At the concert, there were 72 less boys than girls. After ³₅ of the boys and ⁶₇ of the girls had left the hall, there was an equal number of boys and girls that remained behind. Find the number of boys who were present at the concert at first.

	Boys	Girls	Less
Before	5x1 = 5 u	7x2 = 14 u	9 u
Change	- 3x1 = - 3 u	- 6x2 = - 12 u
After	2x1 = 2 u	1x2 = 2 u

Fraction of boys that remained
= 1 - ³₅
= ²₅

Fraction of girls that remained
= 1 - ⁶₇
= ¹₇

The number of boys and girls in the end is the same. Make the number of boys and girls in the end the same. LCM  of 2 and 1 is 2.

^2x1_5x1 Boys = ^1x2_7x2 Girls
²₅ Boys = ²₁₄ Girls

Total boys at first = 5 u
Total girls at first = 14 u

Number of less boys than girls at first
= 14 u - 5 u
= 9 u

9 u = 72

1 u = 72 ÷ 9 = 8

Number of boys at first
= 5 u
= 5 x 8
= 40

Answer(s): 40