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) :
Subtraction is not used in this pattern.
Additional Examples :
Member : What is 7 * 13
Newcomer D : Ummm...109?
Member : Well done! E, What is 2 * 2 * 4?
Newcomer E : I know the answer. It's 8 right?
Member : Yes, correct answer.
Extra Hints (As requested by @El-guest) :
1 * 4 = 8.5
4 * 5 = 20.5
2 * 4 = 10
2 * 2 = 4
Please show the pattern in your answer