Math Problem Statement

Pat is to select six cookies from a tray containing only chocolate chip, oatmeal, and peanut butter cookies. There are at least six of each of the three kinds of cookies on the tray. How many different assortments of six cookies can be selected?

Solution

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Problem 1 (2003 AMC 10A Problem 21)

Question: Pat is to select six cookies from a tray containing only chocolate chip, oatmeal, and peanut butter cookies. There are at least six of each of the three kinds of cookies on the tray. How many different assortments of six cookies can be selected?

Solution: This is a typical "stars and bars" problem, where we distribute 6 cookies into 3 different categories (chocolate chip, oatmeal, and peanut butter). The number of ways to do this is the number of non-negative integer solutions to the equation:

x1+x2+x3=6x_1 + x_2 + x_3 = 6

where x1x_1, x2x_2, and x3x_3 represent the number of each type of cookie chosen. The number of solutions to this equation is given by the stars and bars formula:

(n+k1k1)\binom{n+k-1}{k-1}

where n=6n = 6 (the total number of cookies) and k=3k = 3 (the types of cookies). Thus, the number of ways to select the cookies is:

(6+3131)=(82)=8×72=28\binom{6+3-1}{3-1} = \binom{8}{2} = \frac{8 \times 7}{2} = 28

Answer: The correct number of different assortments of six cookies is 28 (E).


Problem 2 (Mock AIME 2005 Problem 2)

Question: Find the number of 7-digit positive integers that have the property that their digits are in increasing order.

Solution: To solve this problem, consider that we need to select 7 digits from the set {1,2,3,4,5,6,7,8,9}\{1, 2, 3, 4, 5, 6, 7, 8, 9\} such that the digits are in increasing order. Since the digits must be distinct and increasing, this is equivalent to selecting 7 digits from the 9 available digits, and there is only one way to arrange them (in increasing order).

The number of ways to choose 7 digits from 9 is simply:

(97)=(92)=9×82=36\binom{9}{7} = \binom{9}{2} = \frac{9 \times 8}{2} = 36

Answer: The number of such 7-digit integers is 36.


Problem 3

Question: How many ways can you order the integers from 1 to 9 such that no two multiples of 3 are adjacent?

Solution Outline: We have the integers 1,2,,91, 2, \dots, 9, where the multiples of 3 are 3,6,93, 6, 9. The key constraint is that these multiples cannot be adjacent. A possible approach involves calculating the number of ways to arrange the multiples of 3 in a way that separates them by at least one non-multiple of 3.

This can be solved by using a method involving gaps or finding valid distributions of the other numbers in between.


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Here are 5 related questions:

  1. How does the stars and bars method generalize to more complicated distributions?
  2. What is the mathematical reasoning behind the formula for combinations (nk)\binom{n}{k}?
  3. How does one generalize the concept of increasing numbers to different number sets?
  4. Can you find the number of increasing sequences of 6 digits from a set of 8 digits?
  5. How do we calculate the number of non-adjacent multiples of other numbers, like multiples of 4?

Tip: Always visualize the problem by writing out small cases to ensure the approach fits the structure. This can avoid mistakes and simplify reasoning!

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Math Problem Analysis

Mathematical Concepts

Combinatorics
Stars and Bars

Formulas

Binomial coefficient
Stars and bars formula

Theorems

Stars and bars method

Suitable Grade Level

Grades 10-12