Praise:

1) There are some black and white marbles in a bag. If 20 black marbles are removed from the bag, the total number of marbles left will be 7 times the number of black marbles left. If 50 white marbles are removed from the bag, the total number of marbles left will be 5 times the number of black marbles left. How many marbles are there in the bag?

I saw this question a few weeks back using ratio to solve it

case 1: 20 black removed
b:w
1:6

case 2: 50 white are removed
b:w
1:4

Solve
B:W
1:6
+20 -50
----------
1 : 4

Re-writing using common multiples
B:W
4:6
+80 -50
------------
4 : 4


2u-->130
u-->65
7u-->475


the complete solution was found here

http://www.onsponge.com/forum/35-thinkingmathonsponge/859-ratio.html#871