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counting and probability intermediate

Problem

A flag is made of three horizontal strips of fabric, each of a solid color, either red, white, blue or green. If no two adjacent strips can be the same color, how many distinct flags are possible? These two flags are different.

problem
Solution
We can choose the middle color for the flag in 4 ways, then choose the top color in 3 ways, and finally choose the bottom color in 3 ways (the only restriction is that the top and bottom colors are both different from the middle color). This leads to a total of possible flags.
Final answer
36