# Math Help - average problem

1. ## average problem

Five students took a mathematics test. The average mark was 68. If the marks of four boys were 75, 62, 74, and 63, the mark of the fifth boy was:

a. 66 b. 68 c. 76 d. 56 e. None of these

2. Hello,

Let x be the mark of the fifth student.
The definition of "average" is "sum of all marks divided by the number of marks".

This means that if the average is X, we have :

$\frac{\text{1st mark + 2nd mark + 3rd mark + 4th mark + 5th mark}}{5}=X$

Now you know 4 of the 5 marks and you know the average mark. What equation are you going to solve ?

3. (75 + 62 + 74 + 63 + x) / 5 = 68

(274 + x) / 5 = 68

274 + x = 340
x = 340 -274

x = 66

plug 66 in for x in the original equation to check