The ratio of the number of red stamps to green stamps is 7 : 5. After John gives away 10 red stamps and buys another 90 green stamps, there is equal number of red stamps and green stamps. How many stamps does John have now?
| |
Red |
Green |
| Before |
7 u |
5 u |
| Change |
- 10 |
+ 90 |
| After |
7 u - 10 |
5 u + 90 |
The number of red stamps and green stamps in the end is the same.
7 u - 10 = 5 u + 90
7 u - 5 u = 90 + 10
2 u = 100
1 u = 100 ÷ 2 = 50
Number of stamps that John has now
= 7 u - 10 + 5 u + 90
= 7 u + 5 u - 10 + 90
= 12 u + 80
= 12 x 50 + 80
= 600 + 80
= 680
Answer(s): 680
