A group of boys has an average weight of 36 kg. One boy weighing 42 kg leaves the group and another boy weighing 30 kg joins the group. If the average now becomes 35.7 kg, then how many boys are there in the group?
1). 30
2). 32
3). 40
4). 56
Let the number of boys be y
Given that average initially is 36kg
⇒ (sum of age of all boys initially)/y = 36
⇒ 36y = sum of age of all boys initially ----(i)
Now when one boy joins and another leaves
⇒ (sum of age of boys initially - 42 + 30)/y = 35.7 Using (i)
⇒ 36y - 12 = 35.7y
⇒ 0.3y = 12
⇒ y = 40
∴ There are 40 boys in the group