¹₂ of Rael's savings comprised of 20-cent coins and 10-cent coins. Rael used some 20-cent coins to purchase a mini-helicopter. The number of 10-cent coins left was 40% more than the number of 20-cent coins left. Given that Rael had 20 more 10-cent coins than 20-cent coins left in the end, find the amount of money Rael spent on the mini-helicopter.

	20-cent coins	10-cent coins
Before	1x7 = 7 u	1x7 = 7 u
Change	- 2 u
After	5x1 = 5 u	7x1 = 7 u

Fraction of Rael's savings that comprised of 10-cent coins
= 1 - ¹₂
= ¹₂

Number of 10-cents coins in the end in percentage
= 100% + 40%
= 140%

140% = ¹⁴⁰₁₀₀ = ⁷₅

Number of 10-cent coins remains unchanged. Make the number of 10-cent coins the same.  LCM of 1 and 7 is 7.

Number of more 10-cent coins than 20-cent coins
= 7 u - 5 u
= 2 u

2 u = 20

1 u = 20 ÷ 2 = 10

Number of 20-cent coins spent on the mini-helicopter 
= 7 u - 5 u
= 2 u
= 2 x 10
= 20

Amount that Rael spent on the mini-helicopter 
= 20 x 0.2
= $4

Answer(s): $4