Number of Coins

Another question today.

Adam has 46 10-cent, 20-cent and 50-cent coins which add up to $14.60. There were 9 10-cent coins and the rest were 20-cent coins and 50-cent coins. How many 20-cent and 50-cent coins were there?

Solution:

10-cent coins --> $0.90
so 37 coins (20-cent coins + 50-cent coins) --> $14.60 - $0.90 = $13.70

Here we use assumption method to find the answer:

Assume all 37 coins are 50-cent coin --> 37 x $0.50 = $18.50
Difference --> $18.50 - $13.70 = $4.80
Exchange --> $0.50 - $0.20 = $0.30 
Number of 20-cent coin --> $4.80 ÷ $0.30 = 16
Number of 50-cent coin --> 37 - 16 = 21