How many groups of four shirts can you choose from six shirts? determine if the word problem is a permutation or a combination, then evaluate.

When choosing the shirts it doesn't matter what order they're chosen in because at the end you'll have the same four shirts ... so it's a combinations question (rather than a permutations question where the order of choice makes a difference) 
so number of groups = 6C4 = 15 
another way is to evaluate 6P4 / 4! = 360/24 = 15

Physical stimulation can not be used as you carnt ask 1/2 a person a it would be painful