Question

multiple choice answers.

Answer 1

He can buy 3 items in 10 * 9 * 8 ways if order matters.
Since in this case, buying shirts A, B, C is the same as buying shirts A, C, B, then you need to divide by 3 * 2 * 1 to get the total number of combinations of 3 shirts not allowing the same shirts in different orders.

Then he can buy 2 items is 10 * 9 ways taking order into account, but then dividing by 2 * 1, you eliminate the order problem.

The total number of ways is the sum of the two results above.

Answer 2

10 * 9 * 8= 720
3 * 2 * 1= 6

720/6= 120. Correct, or not finish?

Answer 3

Then 10 * 9= 90
90/2= 45

120+45= 165.

Answer 4

165 is the final answer.

Answer 5

Correct.

Answer 6

Also, one last question.

Answer 7

The first question is do you know the difference between permutations and combinations?

Answer 8

If order matters, then it's a permutation.
If order does not matter, then it's a combination.
When you arrange books on a shelf, does order matter or not?