Well, considering you need to use BODMAS (brackets, order, division, multiplication, addition, subtraction) to complete this equation, you would solve it as per below (say the number you are trying to find is 'x'). So:
X / 3 + 8 * 2 - 1 = 45
first of all, lets simplify the equation, using BODMAS:
X / 3 + 16 - 1 = 45
x / 3 + 15 = 45
then remove (or subtract) 15 from both sides:
X / 3 = 30
then, multiply both sides by 3:
X = 90
therefore the number you are looking for is 90!
90 / 3 + 8 * 2 - 1 = 45