Math/Science Initiative - Professor Shai

Shai's Counting Tricks


Counting is part of an area of mathematics called combinatorics which is getting more airplay nowadays at all levels of education.  Combinatorics is intimately tied up with probability, because to calculate many probabilities, you need to count.   There are four major tricks that you will learn on this page.  Each will be illustrated with an example. 

1.  Adding - This or That.
    This is maybe too simple to call a trick.  Let's say you have 5 shirts that you wear in the summer, and your mom buys you 3 new shirts.  How many different ways are there to choose either one of the 5 shorts or one of the 3 shirts?  You have 8 total shirts, so there are 8 ways.  In general, when you see "or" you can think "add".

2.  Multilplying - This and That.
   Let's say you have 5 shirts and 3 hats, and you plan to wear a shirt and a hat today.  How many different outfits can you choose? Since you can wear each of 3 hats with each of the 5 shirts, the total number of outfits is 3 x 5 = 15.  In general when you see "and", you can think "multiply".

3.  Complementing - Counting the Opposite - This minus That.
    How many rolls of two dice do not give you doubles?  There are 36 rolls you can get with two dice (see probability review), and 6 of them are doubles (1-1, 2-2, 3-3, 4-4, 5-5, 6-6).  Therefore 36-6 = 30 of them are not doubles.  When you count the "complement" of something, think "subtraction"

4.  Counting wrong on purpose, with a purpose.
    There are 80 kids in the middle school, and I want to choose two to organize a party.  How many different ways can I choose these two?  The first person I choose can be anyone of the 80 kids, and the second person I choose can be anyone from the 79 that are leftover.  By the multiplying principle, the number of ways I can put these two together is 80 x79 = 6320, because each one of the 80 kids can be joined with each one of the 79.  However this is wrong!  Because for any pair of kids, say Joe and Rachel, I count them once when Rachel is chosen first and Joe is chosen second, and one again when Joe is chosen first and Rachel is chosen second.   So my actual total of 6320 is wrong, but I know just exactly how wrong.  It is twice as big as the real number of pairs.  If I divide 6320 in half I get the correct answer of 3160 pairs.


Here are a number of examples to illustrate the application of these tricks:

1.  At the kosher kiosk at Hershey Park you can buy a hot dog, chicken nuggets, or grilled chicken sandwich.  Along with that you can have plain chips or BBQ chips.  To drink you can get water, coke, sprite, diet coke, or apple juice.  Assuming you order a main course, chips and a drink, how many different meals can you buy at Hershey Park?

Solution:  There are 3 main courses, 2 kinds of chips, and 5 drinks.  Each main course can be combined with each kind of chips, which then can be combined with each kind of drink.  The answer is then 3 x 2 x 5 = 30 different meals.

2.  Harry's pizza shop let's you put up to two toppings on a regular pizza.  The toppings include: mushrooms, onions, peppers, and garlic. They also have a whole wheat crust pizza which comes either plain or with everything.    How many different kinds of pizza can you order?

Solution:  Since there are different rules for the different cursts, let's do the regular crust and the whole wheat crust separately and add the number together.  The whole wheat crust is easy - there are only two kinds of pizzas: plain and everything.  For the regular crust we can have either 0, 1, or 2 toppings.  Let's count each one seperately and add them together.  There is one way to have zero toppings, 4 ways to have one topping, and ...  Two toppings is just a little harder.  There are four toppings (Mushroom, Onion, Peppers, Garlic), so we have four choices for the first topping, and three choices for the second topping.   There are 4 x 3 ways to join these together, gicing 12 different kinds of pizza.  However this method is wrong because it counts every pair twice.  Here is the list:  M-O, M-P, M-G    O-M, O-P, O-G    P-M, P-O, P-G    G-M, G-O, G-P
You should notice that every pair of toppings is counted twice once forward and once backwards.  Therefore the correct number of two topping pizzas is half of this - just 6.  Let's go back and finish the solution:  2 whole wheat + 1 no-topping regular crust + 4 one-topping regular crust +  6 two-topping regular crust = 13 total different pizzas.

3.  You have ten socks in a drawer, half white and half black.  If you choose two socks at random from the drawer, what's the chance that you have a pair of white socks or a pair of black socks?   If your gut instinct is still to say 50/50, then you need to re-read this page and the previous one on probability!

Solution:  The chances of picking two white socks or two black socks equals the number of ways to pick two white socks + the number of ways to pick two black socks, divided by the total number of ways to pick two socks.  Let's handle this in pieces.  The total number of ways to pick two socks from a collection of 10 is 10 x 9 divided by 2 = 45.  (You have seen this idea twice before - when you chose 2 toppings from 4, and when you chose 2 kids from 80).  The number of ways to choose two white socks is 5 x 4 divided by 2 = 10, and the number of ways to choose two black socks is the same 10.  Therefore the total chance to pick a pair of socks from this drawer that match is (10 + 10)/45 = 20/45 = 4/9.

4.  You have 10 socks in a drawer, half white and half black.  If you choose two socks at random from the drawer, what's the chance you do not get a match?  Here we can just take away the 20 matches from the total, to get (45 - 20)/45 = 25/45 = 5/9.

5.  If there are 100 people in town, half men and half women.  Twelve of the men have blue eyes, twelve of the woman have blue eyes, and all the rest have brown eyes.  If two blue eyed people marry, their children are blue-eyed.  If two brown-eyed people marry, their children are brown-eyed.  If a brown-eyed person married a blue-eyed person, then there is a 75% chance that the kid is brown-eyed, and a 25% chance that it is blue-eyed.  If you pick a man and a woman at random, what is the chance that their first child will be blue-eyed?

Solution:  There are 50 men and 50 women, and so there are 50 x 50 = 2500 total possible matches.  Out of these matches, only some will yield blue-eyed babies.  The blue-eyed babies come from the blue-blue marriages, or from brown-blue (or blue-brown) marriages.  Out of the 2500 total marriages, there are 12 x 12 = 144 blue-blue marriages, and all of these will yield blue-eyed children.  Out of the 2500 total marriages, there are 38 x 12 brown-blue marriages + 12 x 38  blue-brown marriages = 912 marriages.  In each of these 912 marriages, there is a 25% chance to have a blue-eyed baby.  That means we expect about 228 of these marriages to have a blue-eyed baby.  Finally adding 144 + 228 we get 372 marriages out of 2500 that are expected have blue-eyed babies.  The probability that the first child of a random marriage is a blue-eyed baby is 372/5000 = 7.44%

6.  "A Real life Problem".  A dad is buying bowling balls for his three sons.  The balls are identical except for the color, and there are five colors.  The dad would like each kid to get a different color, but of course two or more might want the same color.  Is it worth it for the dad to give a speech to the kids about how not everyone might get their first choice on the theory that it is likely to be a fight about the same color, or should he just let them choose and hope that they pick three different colors.  Well this decision should depend on the chance that there will be no fight about the colors.  So assuming that each kid will pick a color at random from the available five colors (choices may not be random but at least we will get a ballpark on the probability), what is the probability that we end up with three different colored balls?

Solution:  This problem is a direct application of  the multiplication principle of counting.  We must count how many ways for the kids to each pick a color, and how many of those ways have three different colors.  Each child has five choices for color, so the total number of ways for all three kids to pick colors is 5 x 5 x 5 = 125.  Let's make sure you understand this.  Call the colors A - E. Then here is the beginning of the list of possible  ways for the kids to pick colors:


That is 25 possibilities.  The same exact list repeats four more times with the first person choosing B instead of A, and then C instead of A, and then D instead of A, and then E instead of A.  That makes 25 x 5 = 125 possible ways to choose colors.

Out of all these ways, how many have three different colors?  Rather than write them all out and count explicitly, there are 5 choices for the first color, and then only 4 choices for the next, and then only three for the last, making 5 x 4 x 3 = 60.  So the chance the kids will live in peace with three different colors is 60/125 = 12/25.  This is just under 50%.  That's like a flip of a coin -- I don't think a preemptive speech is in order!


1.  How many ways can I choose a committee of three people from a group of ten?

2.  If every outfit I wear has one hat, one jacket and one pair of shoes, how many outfits can I wear if all my clothes match and I have 3 hats, 10 pairs of shoes, and 4 jackets?

3.  A Banana Split has three scoops of ice cream.  Each scoop can have any topping; there are 4 flavors of ice cream; and there four different toppings.  How many different Banana Splits can you order?

4.  How many different outcomes can there be when I roll 3 dice?

5.  If I roll three dice, how many ways can I roll a sum of 8 or more?

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