²₉ of the people at a bird park were girls. 80% of the remaining people were boys and the rest were adults. The number of boys was 220 more than the total number of girls and adults. After some boys left the bird park, 50% of the people that remained were boys. How many boys left the bird park?

Girls	Adults	Boys	Total
2x5	7x5		9x5
	1x7	4x7
10	7	28	45

	Girls and adults	Boys
Before	17x1 = 17 u	28x1 = 28 u
Change		- 11 u
After	1x17 = 17 u	1x17 = 17 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 7 and 5 is 35.

50% = ⁵⁰₁₀₀ = ¹₂

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 17 and 1 is 17.

The number of boys was 220 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
= 28 u - 17 u
= 11 u

11 u = 220

1 u = 220 ÷ 11 = 20

Number of boys who left the bird park
= 28 u - 17 u
= 11 u
= 11 x 20
= 220

Answer(s): 220