Math Riddles

Question: What single line can you add to the following statement to make it true?

5+5+5=550

Question: It takes ten men ten hours to build a certain wall. How long does it take five men to build the same wall?

Question: What's the angle between minute hand and hour hand at a quarter past three?

Question: There are 100 coins scattered in a dark room. 90 have heads facing up and 10 are facing tails up. You cannot tell which coins are which. How do you sort the coins into two piles that contain the same number of tails up coins?

Question: What 3 positive numbers give the same result when multiplied and added together?