The average mass of a group of girls was 39 kg. When a boy, Fred, joined the group, the average mass of the group became 33 kg. If Fred 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 |
39 |
33 |
Total value |
39 u |
33 u + 33 |
39 u + 21 = 33 u + 33
39 u - 33 u = 33 - 21
6 u = 12
1 u = 12 ÷ 6 = 2
Number of girls in the group in the end = 2
Answer(s): 2