¹₁₀ of the people at a bird park were girls. 70% of the remaining people were boys and the rest were adults. The number of boys was 1040 more than the total number of girls and adults. After some boys left the bird park, 60% of the people that remained were boys. How many boys left the bird park?

Girls	Adults	Boys	Total
1x10	9x10		10x10
	3x9	7x9
10	27	63	100

	Girls and adults	Boys
Before	37x2 = 74 u	63x2 = 126 u
Change		- 15 u
After	2x37 = 74 u	3x37 = 111 u

70% = ⁷⁰₁₀₀ = ⁷₁₀
Adults : Boys = 3 : 7

The total number of adults and boys at first is repeated.  Make the total number of adults at first the same. LCM of 9 and 10 is 90.

60% = ⁶⁰₁₀₀ = ³₅

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 37 and 2 is 74.

The number of boys was 1040 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
= 126 u - 74 u
= 52 u

52 u = 1040

1 u = 1040 ÷ 52 = 20

Number of boys who left the bird park
= 126 u - 111 u
= 15 u
= 15 x 20
= 300

Answer(s): 300