Andrew , bridget , cindy had 600 stamps altogether. after bridget had given 30 stamps to andrew , bridget had twice as many stamps as cindy and andrew had 20 stamps more than cindy. how many stamps did cindy have?

Bridget:290
Cindy:145
Andrew:165

If you're not very good at maths like me, you estimate. so Bridget twice as many as Cindy so I took something easy to half (also happens to be half the target)
300/2 = 150 (so B=300 and C =150)
So Andrew has 20 more than Cindy. 
150+20 = 170.
B=300, C=150, A=170 
170+300+150=620, ok so we are just twenty over. So we need to get rid of twenty. Take ten off Bridget. Half of ten is five, so we need to take five off Cindy. so now Cindy is 145, well 170 is now to much, we need to take five off here. 
So 10 off Bridget and 5 off each Andrew and Cindy gets us our target of 20. Add it all together you have your answer.   

Its 145 because you can  use the singapore method to figeure it out.