Charlie had 3 times as many black files as brown files. After giving away 4 black files and buying another 10 brown files, he had an equal number of black and brown files. How many black files did Charlie have at first?
| |
Black files |
Brown files |
| Before |
3 u |
1 u |
| Change |
- 4 |
+ 10 |
| After |
3 u - 4 |
1 u + 10 |
The number of black and brown files is the same in the end.
3 u - 4 = 1 u + 10
3 u - 1 u = 10 + 4
2 u = 4 + 10
2 u = 14
1 u = 14 ÷ 2 = 7
Number of black files that Charlie had at first
= 3 u
= 3 x 7
= 21
Answer(s): 21
