kawal27
Using the letters from the word ‘TRAPEZIUM’, how many combinations 5 letters can be formed such that they have exactly 3 consonants?
A)120
B)600
C)3,600
D)4,800
E)7,200
need detail explanation
The right answer is not among the choices. In combinatorics, when someone talks about "combinations", then they specifically mean "sets where order does not matter". It's a word with a precise definition in math. If order matters, you're dealing with a "permutation", not a "combination". You don't need to know these words on actual GMAT questions, but this question is using one of them, and it's using it incorrectly, judging by the answer choices.
We need to pick 3 out of 5 consonants, which we can do in (5)(4)(3)/3! = 10 ways. We need to pick 2 out of 4 vowels, which we can do in (4)(3)/2! = 6 ways. In total, we thus have 10*6 = 60 choices. We should not then arrange our selection in various orders, because the question is asking specifically for "combinations".
I'm assuming we need to pick distinct letters, something else the question needs to be clear about. In general, I'd be wary about using sources for GMAT preparation that misuse basic mathematical terminology.