Tim had 3 coins and David had 15 coins. Their coins consisted of ten-cent coins and twenty-cent coins. When all their twenty-cent coins were exchanged for ten-cent coins, they had a total of 23 coins. How many ten-cent coins did they have at first?
| |
Ten-cent coins |
Twenty-cent coins |
Total |
| Before |
23 - 2 u |
1 u |
18 |
| Change |
+ 2 u |
- 1 u |
+ 1 u |
| After |
23 |
0 |
23 |
Number of coins at first
= 3 + 15
= 18
1 twenty-cent coin can be exchanged for 2 ten-cent coins.
Increase in the number of coins after all the twenty-cent coins were exchanged for ten-cent coins
= 2 u - 1 u
= 1 u
1 u = 23 - 18 = 5
Number of ten-cent coins at first
= 23 - 2 u
= 23 - 2 x 5
= 23 - 10
= 13
Answer(s): 13
