There once was a boy who spent all his time dreaming about getting rich. " I would do anything to be rich". The devil, hearing this, appeared before the boy and offered to make him rich. "See that bridge?" said the devil. Just walk across and I will double your money you have now. In fact, each time you cross I will double your money. There is just one thing you must give me $24 after each crossing. The boy agreed. He crossed the bridge, stopped to count his money and, and sure enough, it had doubled. He paid the devil $24 and crossed again. Again his money doubled. He paid another $24 and crossed a third time. Again his money had doubled, but this time there was only $24 left which he had to pay the devil so he was left with nothing. The devil laughed and vanished.
This is a problem that you need to approach by "working backwards". I am going to do it by drawing a table with 3 rows, one for each time he crossed the bridge. My table has 5 columns:
Now work backwards.
On the third crossing, after he paid the devil $24 he had $0 so he must have had $24 after he crossed the bridge. Since crossing the bridge doubles his money he must have had $12 before he crossed the bridge. Thus the remaining money he had after the second crossing was $12.
Now on crossing 2 he paid the devil $24 and had $12 remaining so he must have had $36 after crossing the bridge. Thus he had $18 before crossing the bridge.
Repeat for the first crossing to find how much money he had to start.
For the second problem, put $30 in the devil column and repeat the steps. For the third problem add a fourth row to the table.Cheers