There were 121 children enrolled in the Yoko Swimming School in 2017. ⁴₁₁ of the children were girls. Additional children joined the swimming school in 2018. For every 4 girls who joined the swimming school, 3 boys joined the swimming school. If there was an equal number of boys and girls in 2018, how many boys joined the school in that year?

	Girls	Boys
Before	44	77
Change	+ 4 u	+ 3 u
After	44 + 4 u	77 + 3 u

Fraction of children who are boys
= 1 - ⁴₁₁
= ⁷₁₁

Number of girls at first
= ⁴₁₁ x 121
= 44

Number of boys at first
= ⁷₁₁ x 121
= 77

Girls who joined : Boys who joined
4 : 3

The number of boys and girls in the end is the same.

44 + 4 u = 77 + 3 u
4 u - 3 u = 77 - 44

1 u = 33

Number of boys who joined the school in that year
= 3 u
= 3 x 33
= 99

Answer(s): 99