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Randomized Coin Flipper: 50-50 Odds For Heads or Tails
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A Summary Of Elementary Probability

Five coins have a total of 32 permutations: H = Head & T = Tail

  HHHTT  
  TTTHH  
  HHTTH  
  TTHHT  
  HHTHT  
  TTHTH  
  HTHTH  
  THTHT  
  HTHHT  
  THTTH  
  HHHHT  
  HTTHH  
  THHTT  
  TTTTH  
  HHHTH  
  TTHHH  
  HHTTT  
  TTTHT  
  HHTHH  
  THHHT  
  HTTTH  
  TTHTT  
  HTHHH  
  THTHH  
  HTHTT  
  THTTT  
   HHHHH   
  THHHH  
  THHTH  
  HTTHT  
  HTTTT  
  TTTTT  
  
  5 H's  
   4 H's 1 T   
  3 H's 2 T's  
  2 H's 3 T's  
  1 H 4 T's  
  5 T's  

The above table of coin permutations is an example of Pascal's Triangle.
It can be expressed algebraically by the following binomial expansion:

(H + T)5   =
H5
+   5 H4 T
+   10 H3 T2
+   10 H2 T3
+   5 HT4
+   T5
5 H's
4 H's 1 T
3 H's 2 T's
2 H's 3 T's
1 H 4 T's
5 T's


The following questions refer to tosses involving all five coins:

1. What is the chance of getting all Heads (i.e. HHHHH)? A glance at the above Pascal's Triangle reveals only one permutation out of 32.

HHHHH = 1/2 X 1/2 X 1/2 X 1/2 X 1/2 = 1/32

2. What is the chance of getting 3 Heads and 2 Tails in that exact order (i.e. HHHTT)? As in the previous example there is only one permutation out of 32 (refer to the top permutation, 3rd column from left).

HHHTT = 1/2 X 1/2 X 1/2 X 1/2 X 1/2 = 1/32

3. What is the chance of getting 3 Heads and 2 Tails in any order? In this example you must consider all possible permutations with 3 Heads and 2 Tails. The 3rd column from left in the above Pascal's Triangle shows 10 permutations out of 32 with 3 Heads and 2 Tails. This is also the probability of having 3 girls and 2 boys when all possible orders are considered. Another way to solve this problem is to multiply 1/32 by the number of permutations: 1/32 X 10 = 10/32 = 5/16.

# of permutations: 5! / 3! 2! = 5 X 4 X 3 X 2 X 1 / 3 X 2 X 1 X 2 X 1 = 120 / 12 = 10

4. What is the chance of getting any permutation except 5 Heads?

5. What is the chance of getting at least one Head?

6. What is the chance of getting one Head?

7. What is the chance of getting no Heads?

8. What is the chance of getting at least 3 Heads?


Probability In Rolling A Pair Of Dice

Miniature dice on a U.S. penny. Each die is 4 mm on a side.
The probability for two 5's is 1/36 or 3/36 for total of 10 dots.


A total of 36 different combinations: 6 X 6 = 36

  Die  
R e d  
G
r
e
e
n
Number
of Dots
    2    
    3    
    4    
    5    
    6    
    7    
3
4
5
6
7
8
4
5
6
7
8
9
5
6
7
8
9
10
6
7
8
9
10
11
7
8
9
10
11
12

There are 36 possible combinations when rolling a pair of dice. Of all the combinations of dots, seven is the most likely number. The probability of coming up with a seven is 1/6 (one out of six rolls) because six combinations of red and green dice add up to seven (6/36 = 1/6). The chance of getting a two (snake eyes) is only 1/36.


Probability In Drawing A Royal Flush

Drawing Cards In Order From A Randomly Shuffled Deck

A royal flush is an ace, king, queen, jack and ten in the same suit. If the cards are drawn in that order from a deck of 52 cards, then the chance of the first card being an ace is 4/52 since there are four aces in the deck and no suit has been specified. The chance of the second card being a king of the same suit is 1/51 because there is only one king of that suit and there are 51 cards left in the deck. The following mathematical calculation shows the unlikely probability of drawing a royal flush in order from a deck of 52 cards:

4/52 X 1/51 X 1/50 X 1/49 X 1/48 = 4 / 311,875,200
[This is roughly one chance out of 78 million.]

It is more likely to draw a royal flush from a deck of 52 cards if the cards can be drawn in any order. In this case the first card is 20/52 because there are four suits (club, spade, heart & diamond) and five different cards (ace, king, queen, jack & ten) to choose from. Then you must consider all the possible orders (permutations) that the cards can be drawn, such as ace-king-queen-jack-ten, ten-jack-queen-king-ace, ace-jack-ten-king-queen, etc. The number of permutations can be calculated from five factorial or 5! = 5 X 4 X 3 X 2 X 1 = 120:

20/52 X 1/51 X 1/50 X 1/49 X 1/48 X 120 = 2400 / 311,875,200
[This is roughly one chance out of 130 thousand.]


Probability Of Winning The California Lottery

Picking Six Out Of 53 Numbers With A Single Ticket

The probability of one winner picking all six correct numbers out of 53 numbers is similar to the above question about drawing a royal flush from a deck of cards. The first number is 1/53 because it is one number out of 53 possibilities. The second number is 1/52 because the total number is now reduced by one to 52. Since there are different permutations for the six numbers, you must multiply the total probability by six factorial or 6! = 6 X 5 x 4 X 3 X 2 X 1 = 720:

1/53 X 1/52 X 1/51 X 1/50 X 1/49 X 1/48 X 720 = 720 / 16,529,385,000
[This is roughly one chance out of 23 million.]

According to B. Siskin and J. Staller (What are the Chances?, Crown Publishers, Inc., New York, 1989), the chance of being struck by lightning in your lifetime is one in 600,000. Compare this with the chance of winning the state lottery (one in 23 million)! Of course, the chance of being struck by lightning is increased significantly if you stand on a barren mountain summit (above timberline) holding a metal rod during a thunder storm.


Probability Of Sex Determination In Humans

Sex Determination Is More Complicated Than Tossing A Coin

Fertilization (syngamy) is the fusion of two haploid gametes (the sperm and the egg) to form a diploid (2n) zygote. This is how the chromosome number in a life cycle changes from haploid (n) to diploid (2n). Since human males produce X-bearing and Y-bearing sperm, and human females produce only X-bearing eggs, the gametes combine randomly according to the following table:

Gametes
X-bearing sperm
Y-bearing sperm
X-bearing egg
XX
XY

The male (XY) and female (XX) offspring in the above table are in a 50-50 ratio with an equal number of boys and girls. Therefore, the chance of having a boy is 1/2 or 50% and the chance of a girl is also 1/2 or 50%. This ratio can be demonstrated by tossing a coin many times and keeping track of the number of heads and tails. If enough tosses are made, the number of heads and tails should be very close to 50-50.

Go Back To The Random Coinflip
  Probability With Tosses Of 5 Coins  

Unfortunately in biology, sex ratios in humans are not that easily explained. In the United States, there is a slightly better chance of having a boy, about 105 males to 100 females. There are a number of hypotheses (tentative explanations) for this unequal birth ratio, most of which are probably not accurate. If this unequal birth ratio is the result of a greater number of male conceptions, then perhaps the Y-sperm has a slightly better advantage in reaching the egg or penetrating the barrier of follicle cells around the egg, possibly by the smaller size of its head and faster speed. According to some references, muscular contractions and ciliary currents within the female reproductive tract are primarily responsible for transporting the sperm to the egg. In vitro, sperm can swim about 3 mm per minute, but within the vagina and oviduct (in vivo) they can travel about 5 mm per minute. Since the speed is additive, a faster Y-sperm could potentially still win the race to the egg. It has been demonstrated that Y-bearing sperm do not live as long as X-bearing sperm, so the time of ovulation and sexual intercourse could be a factor in determining the sex of a child. By the time the sperm reach the upper part of the fallopian tube where fertilization occurs, most of the Y-sperm may have already died. In this case when the egg is released from the ovary, the odds would be favor fertilization by an X-bearing sperm resulting in a girl. Another hypothesis for unequal sex ratios is an unequal number of X-bearing and Y-bearing sperm in the man's semen. Some references state that this condition might be hereditary, but it is not clearly explained in more scholarly texts.

A more plausible explanation for unequal birth ratios may involve the rejection of an embryo by the mother's antibodies. Conceptions may be approximately equal, but a greater loss of female embryos early in the gestation period may account for slightly more male births. The mother has two X-chromosomes, one from her mother and one from her father. Perhaps the X-chromosome from her father carries a gene that sensitizes her against the female fetus. In other words she develops antibodies against proteins of the female fetus but not the male fetus. Remember that these are only hypotheses at this time and should not be accepted as the final answer. When more testing is done, perhaps one or more of these hypotheses will become a widely-accepted theory.


Puzzle Games Based On Arrangements (Permutations) On The Sides Of Cubes

Front Center: Instant Insanity. 4 blocks whose faces (sides) have 4 colors (red, green, blue & white). The object is to stack blocks so that all 4 colors show on each side of stack. Each block can be arranged in 24 different positions (6 sides x 4 colors = 24). The number of possible arrangements is 244 = 331, 776; however, the solved puzzle has 8 orientations so you must divide this total number by 8: 331,776 /8 = 41,472. The solution does not require all the different orders of the arrangements, but if you want them, then multiply the number of arrangements by 4 factorial: 41,472 x 4! = 995,328.

Front Right: Rubik's Cube: Each of the 6 faces (sides) of cube contains 9 stickers, each one of 6 colors (red, green, blue, white, orange & yellow). A pivot mechanism enables each vertical or horizontal row of 3 stickers to turn independently, thus mixing up the colors. The object of puzzle is to return each side to a single color of 9 stickers. There are 8! (40,320) ways to arrange the corner cubes. 7 corner cubes can be oriented independently, and the orientation of the 8th depends on the preceeding 7, giving 37 (2,187) possibilities. There are 12! /2 (239,500,800) ways to arrange the edges, since an odd permutation of the corners implies an odd permutation of the edges as well. 11 edges can be flipped independently, with the flip of the 12th depending on the preceeding ones, giving 211 (2,048) possibilities.

Number of arrangements: 8! x 37 x 12! /2 x 211 = 43,252,003,274,489,856,000 or approximately 43 quintillion. According to Wikipedia, this number of Rubik's cubes could cover the earth's surface more than 275 times!