In a coin box, the ratio of the number of red marbles to the number of brown marbles was 1 : 3. Each time, 3 red and 7 brown marbles were taken out of the coin box. A while later, only 140 brown marbles were left in the coin box. What was the total number of marbles in the coin box at first?

	Red marbles	Brown marbles
Before	1x3 = 3 u	3x3 = 9 u
Change	- 3 u	- 7 u
After	0	140

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

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

Number of brown marbles in the end
= 9 u - 7 u
= 2 u

2 u = 140

1 u = 140 ÷ 2 = 70

Total number of marbles in the coin box at first
= 3 u + 9 u
= 12 u
= 12 x 70
= 840

Answer: 840