In a bottle, the ratio of the number of yellow marbles to the number of grey marbles was 1 : 3. Each time, 3 yellow and 6 grey marbles were taken out of the bottle. A while later, only 120 grey marbles were left in the bottle. What was the total number of marbles in the bottle at first?

	Yellow marbles	Grey marbles
Before	1x3 = 3 u	3x3 = 9 u
Change	- 3 u	- 6 u
After	0	120

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

Make the number of yellow marbles at first and taken out the same.  LCM of 1 and 3 is 3.

Number of grey marbles in the end
= 9 u - 6 u
= 3 u

3 u = 120

1 u = 120 ÷ 3 = 40

Total number of marbles in the bottle at first
= 3 u + 9 u
= 12 u
= 12 x 40
= 480

Answer: 480