If a#b = ab – b + b², then 3#5 =

A. 2
B. 8
C. 15
D. 21
E. 35

(E) On test day, the # symbol or any other such symbol in a question will represent a function.
In this case, a#b = ab – b + b² is a function. Simply substitute a = 3 and b = 5 into the formula and calculate:
3#5 = 3(5) – 5 + 5² = 15 – 5 + 25 = 35.
The correct answer is choice (E).
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Shouldn't you add before you subtract?

Subtraction is the same as addition of the value, multiplied by (-1).

15 – 5 + 25 = 15 + (-5) + 25

So the order, in which you do addition and subtraction, is up to you.

POSSIBLE:
15 – 5 + 25 = (15 – 5) + 25 = 10 + 25 = 35
or
15 – 5 + 25 = 15 + (-5 + 25) = 15 + 20 = 35

But if you group elements and want the "-" sign to be in front of the brackets, don't forget to multiply all the terms, that go into the brackets, by -1.

WRONG:
15 – 5 + 25 = 15 – (5 + 25) = 15 – 30 = -15

RIGHT:
15 – 5 + 25 = 15 – (5 – 25) = 15 – (-20) = 15 + 20 = 35