⁵₈ of Riordan's savings comprised of 5-cent coins and 50-cent coins. Riordan used some 5-cent coins to purchase a mini-helicopter. The number of 50-cent coins left was 40% more than the number of 5-cent coins left. Given that Riordan had 120 more 50-cent coins than 5-cent coins left in the end, find the amount of money Riordan spent on the mini-helicopter.

	5-cent coins	50-cent coins
Before	5x7 = 35 u	3x7 = 21 u
Change	- 20 u
After	5x3 = 15 u	7x3 = 21 u

Fraction of Riordan's savings that comprised of 50-cent coins
= 1 - ⁵₈
= ³₈

Number of 50-cents coins in the end in percentage
= 100% + 40%
= 140%

140% = ¹⁴⁰₁₀₀ = ⁷₅

Number of 50-cent coins remains unchanged. Make the number of 50-cent coins the same.  LCM of 3 and 7 is 21.

Number of more 50-cent coins than 5-cent coins
= 21 u - 15 u
= 6 u

6 u = 120

1 u = 120 ÷ 6 = 20

Number of 5-cent coins spent on the mini-helicopter 
= 35 u - 15 u
= 20 u
= 20 x 20
= 400

Amount that Riordan spent on the mini-helicopter 
= 400 x 0.05
= $20

Answer(s): $20