Math Riddles

Question: When is 1500 plus 20 and 1600 minus 40 the same thing?

Question: In a certain society any time somebody commits a serious crime they must be shot at twice with a 6 bullet revolver. The revolver only has two bullets in it though, both of them right next to each other. They spin the revolver once and shoot the gun. If there was no bullet in that chamber they give the prisoner the option to either spin the chamber again or just shoot again. If the first shot is a blank, should the prisoner ask for the revolver to be spun or should they choose that it be shot again?

Question: You have two buckets: one with only white marbles and one with only black marbles. The two buckets have the same number of marbles.

How can you rearrange the marbles to maximize your chances of grabbing a white marble from each?

Question: Everyday a peasant must pay the king one pound of gold and leave it on a collection plate in front of his house. Every morning a guard comes by to make sure he has put a pound of gold on the plate. The king collects the gold every six days from the plate. If the peasant only has one six pound block of gold, how can he make only two parallel cuts and still follow the kings rules each day?

Question: You have three coins. One always comes up heads, one always comes up tails, and one is just a regular coin (has equal change of heads or tails). If you pick one of the coins randomly and flip it twice and get heads twice, what is the chance of flipping heads again?