I was about to try to join the famous Math club at my school when I stopped to hear my classmates doing the same. I didn't know that newcomers had to take a simple test to be qualified until my they tried it. It went like this:

Member : What is 1 * 3

Newcomer A : Is it 5?

Member : Correct, you may go in. B, what is 1 * 4 * 5?

Newcomer B : It's gotta be 14, right?

Member : Indeed, you may also go in. C, what is the result of 2 * 4 * 6 * 7 * 10?

Newcomer C : Is it 40?

Member : Wrong, but you are so close. Try again next time.

After hearing that conversation, I went to the member and said that I would like to join the club too. The member then asks, "Very well, tell me the result of 3 * 12 * 15 and if you are correct, then I will let you in."

What should I answer?

To clarify :

The symbol (*) does not mean to multiply.

Hint (Usefulness Level : 1) :

Observe the amount of numbers in the equations

Hint (Usefulness Level : 2) :

The pattern only uses 3 kinds of operations.

Hint (Usefulness Level : 3) :

Addition and Subtraction are not used in this pattern.

Will add more examples in the future if nobody gets it right.