75 pupils sat for a Science Test. The average score was 58 marks. If the average score of the girls was 62 marks and the average score of the boys was 56 marks, how many boys sat for the test?

Answer 1

Let g=number of girls,
and assuming all data given are exact (i.e. no rounding involved), then
g*62+(75-g)*58=56
Solve for g.

Answer 2

Looks to me like
g*62 + (75-g)*56 = 75*58

Or, since we're looking for the number of boys, not girls,

56b + 62(75-b) = 58*75

Answer 3

Right, there was a typo! Thought I corrected it. Thanks Steve!
Should read:
g*62+(75-g)*56=75*58