PSLE
Ethan had 9 bags of marbles. At first, each of the bags contained the same number of marbles. He took 15 marbles from each bag. After that, the total number of marbles left in the 9 bags was equal to the total number of marbles in 4 of the bags at first. What was the number of marbles in each bag at first?

	4 bags at first	Total bags in the end
Number	4	9
Value	1 u + 15	1 u
Total value	4 u + 60	9 u

Number of marbles in each bag in the end = 1 u
Number of marbles in each bag at first = 1 u + 15

Number of marbles in 4 bags at first 
= 4 x (1 u + 15)
= 4 u + 60

Total number of marbles left in the 9 bags was equal to total number of marbles in 4 bags at first.

9 u = 4 u + 60 
9 u - 4 u = 60
5 u = 60

1 u = 60 ÷ 5 = 12

Number of marbles in each bag at first
= 1 u + 15
= 12 + 15
= 27

Answer(s): 27