PSLE
Jack had 9 bottles of balls. At first, each of the bottles contained the same number of balls. He took 15 balls from each bottle. After that, the total number of balls left in the 9 bottles was equal to the total number of balls in 4 of the bottles at first. What was the number of balls in each bottle at first?

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

Number of balls in each bottle in the end = 1 u
Number of balls in each bottle at first = 1 u + 15

Number of balls in 4 bottles at first 
= 4 x (1 u + 15)
= 4 u + 60

Total number of balls left in the 9 bottles was equal to total number of balls in 4 bottles at first.

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

1 u = 60 ÷ 5 = 12

Number of balls in each bottle at first
= 1 u + 15
= 12 + 15
= 27

Answer(s): 27