PSLE
Bryan had 6 boxes of balls. At first, each of the boxes contained the same number of balls. He took 10 balls from each box. After that, the total number of balls left in the 6 boxes was equal to the total number of balls in 4 of the boxes at first. What was the number of balls in each box at first?

	4 boxes at first	Total boxes in the end
Number	4	6
Value	1 u + 10	1 u
Total value	4 u + 40	6 u

Number of balls in each box in the end = 1 u
Number of balls in each box at first = 1 u + 10

Number of balls in 4 boxes at first 
= 4 x (1 u + 10)
= 4 u + 40

Total number of balls left in the 6 boxes was equal to total number of balls in 4 boxes at first.

6 u = 4 u + 40 
6 u - 4 u = 40
2 u = 40

1 u = 40 ÷ 2 = 20

Number of balls in each box at first
= 1 u + 10
= 20 + 10
= 30

Answer(s): 30