⁵₉ of Billy's savings comprised of 20-cent coins and 50-cent coins. Billy used some 20-cent coins to purchase a mini-helicopter. The number of 50-cent coins left was 25% more than the number of 20-cent coins left. Given that Billy had 80 more 50-cent coins than 20-cent coins left in the end, find the amount of money Billy spent on the mini-helicopter.

	20-cent coins	50-cent coins
Before	5x5 = 25 u	4x5 = 20 u
Change	- 9 u
After	4x4 = 16 u	5x4 = 20 u

Fraction of Billy's savings that comprised of 50-cent coins
= 1 - ⁵₉
= ⁴₉

Number of 50-cents coins in the end in percentage
= 100% + 25%
= 125%

125% = ¹²⁵₁₀₀ = ⁵₄

Number of 50-cent coins remains unchanged. Make the number of 50-cent coins the same.  LCM of 4 and 5 is 20.

Number of more 50-cent coins than 20-cent coins
= 20 u - 16 u
= 4 u

4 u = 80

1 u = 80 ÷ 4 = 20

Number of 20-cent coins spent on the mini-helicopter 
= 25 u - 16 u
= 9 u
= 9 x 20
= 180

Amount that Billy spent on the mini-helicopter 
= 180 x 0.2
= $36

Answer(s): $36