¹₉ of the visitors at a zoo were girls. 90% of the remaining visitors were boys and the rest were adults. The total number of girls and adults is 189 less than the number of boys. After some boys entered the zoo, 90% of the visitors that remained were boys. How many boys entered the zoo?

Girls	Adults	Boys	Total
1x5	8x5		9x5
	1x4	9x4
5	4	36	45

	Girls and adults	Boys
Before	9x1 = 9 u	36x1 = 36 u
Change		+ 45 u
After	1x9 = 9 u	9x9 = 81 u

90% = ⁹⁰₁₀₀ = ⁹₁₀
Adults : Boys = 1 : 9

The total number of adults and boys at first is repeated.  Make the total number of adults at first the same. LCM of 8 and 10 is 40.

90% = ⁹⁰₁₀₀ = ⁹₁₀

When some boys entered, the total number of girls and adults remains unchanged. Make the total number of girls and adults the same. LCM of 9 and 1 is 9.

The total number of girls and adults is 189 less than the number of boys.

Number of less girls and adults than the number of boys at first
= 36 u - 9 u
= 27 u

27 u = 189

1 u = 189 ÷ 27 = 7

Number of boys who entered the zoo
= 81 u - 36 u
= 45 u
= 45 x 7
= 315

Answer(s): 315