This is a tricky one. The key is :tianzhu wrote: 3) There are 600 children in Team A and 30% of them are boys.

There are 400 children in Team B and 60% of them are boys.

After some children are transferred from Team B to Team A, 40% of the children in Team A and 60% of the children in Team B are boys.

How many children are transferred from Team B to Team A?

**The total number of boys before and after the movement is the same.**

Total boys = (30% x 600) + (60% x 400) = 420

Total girls = (70% X 600) + (40% x 400) = 580

Total = 1000

After movement, in team A

Boys : Girls

= 4 : 6

= 2 : 3

In team A, there are 5units of children.

In team B, there will be 1000 - 5 units of children.

Team B ratio,

Boys : Girls

= 6 : 4

= 3 : 2

= 3/5(1000 -5u) : 2/5(1000 -5u)

2u of boys in team A + 3/5(1000 - 5u) of boys in team B = 420

2u + 600 - 3u = 420

1u = 180

# of boys in team A after movement = 2u = 360

# of girls in team A after movement = 3u = 540

Total in team A = 900.

# of children transferred from team B to team A = 900 - 600 = 300.