Math Riddles

Question: When i was 4 my sister was half my age. Now I am 100. How old is my sister now?

Question: 123-----111213-----31121113-----? What is the next term in the sequence?

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: If I am buying 9 for $1, 18 for $2, 125 for $3, and 1222 for $4, then what am I buying?

Question: If a mountain climber climbed up a mountain one day from midnight to midnight the next day and does the same thing the following day when coming down the mountain, would there be a time at which he was at the same position on the mountain both day?