Answer:
5 shirts
Explanation:
Think of this question like this, where each shirt option (5 options) has a number:
Shirt #1, Shirt #2, Shirt #3, Shirt #4, and lastly Shirt #5
Without using much math, the answer is 5 shirts because every possible outcome (of the 4 shirts Michael must pick to bring on vacation) will leave exactly one shirt that he will not be able to pack. The only possible outcomes that would fit this question would be:
1) Shirt #1, Shirt #2, Shirt #3, and Shirt #4
2) Shirt #2, Shirt #3, Shirt #4, and Shirt #5
3) Shirt #3, Shirt #4, Shirt #5, and Shirt #1
4) Shirt #4, Shirt #5, Shirt #1, and Shirt #2
5) Shirt #5, Shirt #1, Shirt #2, and Shirt #3