PSLE
Paul had 7 boxes of marbles. At first, each of the boxes contained the same number of marbles. He took 12 marbles from each box. After that, the total number of marbles left in the 7 boxes was equal to the total number of marbles in 4 of the boxes at first. What was the number of marbles in each box in the end?

	4 boxes at first	Total boxes in the end
Number	4	7
Value	1 u + 12	1 u
Total value	4 u + 48	7 u

Number of marbles in each box in the end = 1 u
Number of marbles in each box at first = 1 u + 12

Number of marbles in 4 boxes at first 
= 4 x (1 u + 12)
= 4 u + 48

Total number of marbles left in the 7 boxes was equal to total number of marbles in 4 boxes at first.

7 u = 4 u + 48 
7 u - 4 u = 48
3 u = 48

1 u = 48 ÷ 3 = 16

Number of marbles in each box in the end = 16

Answer(s): 16