¹₉ of the people at a museum were girls. 80% of the remaining people were boys and the rest were adults. The number of boys was 285 more than the total number of girls and adults. After some boys left the museum, 70% of the people that remained were boys. How many boys left the museum?

Girls	Adults	Boys	Total
1x5	8x5		9x5
	1x8	4x8
5	8	32	45

	Girls and adults	Boys
Before	13x3 = 39 u	32x3 = 96 u
Change		- 5 u
After	3x13 = 39 u	7x13 = 91 u

80% = ⁸⁰₁₀₀ = ⁴₅
Adults : Boys = 1 : 4

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 5 is 40.

70% = ⁷⁰₁₀₀ = ⁷₁₀

When some boys left, the total number of girls and adults remains unchanged. Make the total number of girls and adults the same. LCM of 13 and 3 is 39.

The number of boys was 285 more than the total number of girls and adults at first.

Number of more boys than the total number of girls and adults at first
= 96 u - 39 u
= 57 u

57 u = 285

1 u = 285 ÷ 57 = 5

Number of boys who left the museum
= 96 u - 91 u
= 5 u
= 5 x 5
= 25

Answer(s): 25