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## Homework Statement

see attachment

## Homework Equations

Combinations and Arithmetic progression formulae

Nth term = 4+(N-1)(3)

## The Attempt at a Solution

After recognizing that this is an arithmetic progression, I calculated the number of terms as : 15

Then , as we have to find out, in simple terms, the number of sets of three different numbers from these 15 numbers .

I applied the combinations rule :

15!/(3!*12!)

But the answer is clearly not among any of the choices.

Please help explain to me where did I go wrong and also recommend me some sources (or topics) I could study to solve such questions.

Thanks