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

To solve the problem of determining the number of different assortments of eight cookies from three types (chocolate chip, oatmeal, and peanut butter) where there are at least four of each type, we can use the "stars and bars" method, a common combinatorial technique.

The situation can be described using the equation:
\[ x_1 + x_2 + x_3 = 8 \]

where:
- \( x_1 \) is the number of chocolate chip cookies,
- \( x_2 \) is the number of oatmeal cookies,
- \( x_3 \) is the number of peanut butter cookies,
and each \( x_i \) must be a non-negative integer. Since there are at least four of each cookie, \( x_1, x_2, \) and \( x_3 \) must be at least 0.

In the stars and bars method, the number of solutions to the equation \( x_1 + x_2 + x_3 = n \) is given by the binomial coefficient:

\[
\binom{n + k - 1}{k - 1}
\]

where \( n \) is the total number of items (cookies in this case) to distribute, and \( k \) is the number of categories (types of cookies).

Here, \( n = 8 \) and \( k = 3 \), so we have:

\[
\binom{8 + 3 - 1}{3 - 1} = \binom{10}{2}
\]

Now we calculate \( \binom{10}{2} \):

\[
\binom{10}{2} = \frac{10 \times 9}{2 \times 1} = 45
\]

Therefore, there are 45 different assortments of eight cookies that can be selected from the tray.

**Summary**: The total number of different assortments of eight cookies from the three types is 45.

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Would you like more details or have any questions? 

Here are some related questions that expand upon this concept:

1. How would the solution change if there were restrictions on the number of each type of cookie?
2. What if the tray contained more than three types of cookies?
3. How does the stars and bars method apply to more than three variables?
4. What is the general formula for the number of solutions in the stars and bars problem?
5. How do permutations differ from combinations in this context?
6. What happens if the total number of cookies to be selected changes?
7. Can you explain the stars and bars method in a different context, such as distributing money or other items?
8. How would the solution be affected if cookies were distinguishable instead of indistinguishable?

**Tip**: When dealing with combinatorial problems, always check the constraints and ensure all conditions are accounted for before applying formulas.

Learn how to calculate the number of different assortments of cookies using combinatorial methods. Solve the problem of selecting eight cookies from a tray containing chocolate chip, oatmeal, and peanut butter cookies with at least four of each type. Includes step-by-step solution and application of the stars and bars method. Suitable for Grades 9-12 students and enthusiasts of combinatorial mathematics.

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