³₅ of David's savings comprised of 10-cent coins and 20-cent coins. David used some 10-cent coins to purchase a marble. The number of 20-cent coins left was 25% more than the number of 10-cent coins left. Given that David had 40 more 20-cent coins than 10-cent coins left in the end, find the amount of money David spent on the marble.

	10-cent coins	20-cent coins
Before	3x5 = 15 u	2x5 = 10 u
Change	- 7 u
After	4x2 = 8 u	5x2 = 10 u

Fraction of David's savings that comprised of 20-cent coins
= 1 - ³₅
= ²₅

Number of 20-cents coins in the end in percentage
= 100% + 25%
= 125%

125% = ¹²⁵₁₀₀ = ⁵₄

Number of 20-cent coins remains unchanged. Make the number of 20-cent coins the same.  LCM of 2 and 5 is 10.

Number of more 20-cent coins than 10-cent coins
= 10 u - 8 u
= 2 u

2 u = 40

1 u = 40 ÷ 2 = 20

Number of 10-cent coins spent on the marble 
= 15 u - 8 u
= 7 u
= 7 x 20
= 140

Amount that David spent on the marble 
= 140 x 0.1
= $14

Answer(s): $14