⁵₇ of Tommy's savings comprised of 10-cent coins and 20-cent coins. Tommy used some 10-cent coins to purchase a mini-helicopter. The number of 20-cent coins left was 40% more than the number of 10-cent coins left. Given that Tommy had 40 more 20-cent coins than 10-cent coins left in the end, find the amount of money Tommy spent on the mini-helicopter.

	10-cent coins	20-cent coins
Before	5x7 = 35 u	2x7 = 14 u
Change	- 25 u
After	5x2 = 10 u	7x2 = 14 u

Fraction of Tommy's savings that comprised of 20-cent coins
= 1 - ⁵₇
= ²₇

Number of 20-cents coins in the end in percentage
= 100% + 40%
= 140%

140% = ¹⁴⁰₁₀₀ = ⁷₅

Number of 20-cent coins remains unchanged. Make the number of 20-cent coins the same.  LCM of 2 and 7 is 14.

Number of more 20-cent coins than 10-cent coins
= 14 u - 10 u
= 4 u

4 u = 40

1 u = 40 ÷ 4 = 10

Number of 10-cent coins spent on the mini-helicopter 
= 35 u - 10 u
= 25 u
= 25 x 10
= 250

Amount that Tommy spent on the mini-helicopter 
= 250 x 0.1
= $25

Answer(s): $25