George had 3 coins and Justin had 16 coins. Their coins consisted of twenty-cent coins and fifty-cent coins. When all their fifty-cent coins were exchanged for twenty-cent coins, they had a total of 28 coins. How many twenty-cent coins did they have at first?
| |
Twenty-cent coins |
Fifty-cent coins |
Total |
| Before |
28 - 5 u |
2 u |
19 |
| Change |
+ 5 u |
- 2 u |
+ 3 u |
| After |
28 |
0 |
28 |
Number of coins at first
= 3 + 16
= 19
2 fifty-cent coins can be exchanged for 5 twenty-cent coins.
Increase in the number of coins after all the fifty-cent coins were exchanged for twenty-cent coins
= 5 u - 2 u
= 3 u
3 u = 28 - 19
3 u = 9
1 u = 9 ÷ 3 = 3
Number of twenty-cent coins at first
= 28 - 5 u
= 28 - 5 x 3
= 28 - 15
= 13
Answer(s): 13
