The average weight of a group of boys is 30 kg. After a boy of weight 35 kg joins the group, the average weight of the group goes up by 1 kg. Find the number of boys in the group originally?
- 4
- 5
- 6
- 7
Explanation
Let the original number of boys be n.
Original average = 30 kg
New average after adding boy = 31 kg
Weight of new boy = 35 kg
Total weight originally = 30n
Total weight after adding = 30n + 35
New average = (30n + 35) / (n + 1) = 31
So,
(30n + 35) = 31(n + 1)
30n + 35 = 31n + 31
35 - 31 = 31n - 30n
4 = n
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