The average mass of a group of girls was 36 kg. When a boy, Will, joined the group, the average mass of the group became 31 kg. If Will had a mass of 21 kg, how many girls were there in the group in the end?
|
At first |
In the end |
Number |
1 u |
1 u + 1 |
Value |
36 |
31 |
Total value |
36 u |
31 u + 31 |
36 u + 21 = 31 u + 31
36 u - 31 u = 31 - 21
5 u = 10
1 u = 10 ÷ 5 = 2
Number of girls in the group in the end = 2
Answer(s): 2