There was an equal number of green and black balls in a packet at first. After 21 black balls were taken out and 39 green balls were added into the packet, there was
13 as many black balls as green balls. Find the number of black balls in the packet at first.
| |
Green balls |
Black balls |
| Before |
3 u - 39 |
1 u + 21 |
| Change |
+ 39 |
- 21 |
| After |
3 u |
1 u |
The number of green and black balls in the packet is the same at first.
3 u - 39 = 1 u + 21
3 u - 1 u = 21 + 39
2 u = 60
1 u = 60 ÷ 2 = 30
Number of black balls at first
= 1 u + 21
= 30 + 21
= 51
Answer(s)51
