Mr Koh went to 3 shops with some money. At each shop, he spent $10 more than half of what he had when he entered. He did not have any money left in the end. How much did Mr Koh have before he entered the first shop?
look at the money spent at each shop, starting with x
1: x/2 + 10, leaving x/2 - 10
2: x/4 - 5 + 10, leaving x/4 - 15
3: x/8 - 15/2 + 10, leaving 0
So, x = 7x/8 + 30 -5 - 15/2
x = 140
check:
at shop 1, spent 80, leaving 60
at shop2, spent 40, leaving 20
at shop 3, spent 20, leaving 0