In a bottle, the ratio of the number of brown balls to the number of green balls was 1 : 3. Each time, 3 brown and 7 green balls were taken out of the bottle. A while later, only 162 green balls were left in the bottle. What was the total number of balls in the bottle at first?

	Brown balls	Green balls
Before	1x3 = 3 u	3x3 = 9 u
Change	- 3 u	- 7 u
After	0	162

Since there is no brown balls left, this means that the number of brown marbles at first is the same as the number of brown marbles taken out.  

Make the number of brown balls at first and taken out the same.  LCM of 1 and 3 is 3.

Number of green balls in the end
= 9 u - 7 u
= 2 u

2 u = 162

1 u = 162 ÷ 2 = 81

Total number of balls in the bottle at first
= 3 u + 9 u
= 12 u
= 12 x 81
= 972

Answer: 972