The ratio of the number of red erasers to blue erasers is 7 : 5. After Billy gives away 10 red erasers and buys another 20 blue erasers, there is equal number of red erasers and blue erasers. How many erasers does Billy have now?
|
Red |
Blue |
Before |
7 u |
5 u |
Change |
- 10 |
+ 20 |
After |
7 u - 10 |
5 u + 20 |
The number of red erasers and blue erasers in the end is the same.
7 u - 10 = 5 u + 20
7 u - 5 u = 20 + 10
2 u = 30
1 u = 30 ÷ 2 = 15
Number of erasers that Billy has now
= 7 u - 10 + 5 u + 20
= 7 u + 5 u - 10 + 20
= 12 u + 10
= 12 x 15 + 10
= 180 + 10
= 190
Answer(s): 190