Question

15 + 3 • 4 – 8 ÷ 2

Answer 1

Do you know the Order of Operations?

Answer 2

They are usually shown as PEMDAS:

Parentheses
Exponents
Multiplication or Division, whichever one comes first from left to right
Addition or Subtraction, whichever one comes first from left to right

Answer 3

Let's apply that knowledge to our problem.
1) First we will multiply

15 + (3 * 4 = 12) - 8 / 2

15 + 12 - 8 / 2

2) Now we divide

15 + 12 ( -8 / 2 )

15 + 12 - 4

3) Now we add

27 - 4

4) Now we subtract

23