Make 42 out of 2,3,4, and 9.
You may use each number once and only once using the arithmetic operations +,-,* and /.

Clocks

1/ The hour and minute hands of a clock are superimposed at 12:00. When will they next be superimposed (I don't mean lined up as they are at 6:00)?
2/ Which has a better chance of giving the right time: a clock that has stopped or one that loses a minute every day?

An old one but a good one.

Bob is 104. He is twice as old as Alice was when Bob was as old as Alice is now. How old is Alice?

Rope around the planet

Imagine a rope that fits snugly all around a planet like a ring on a person's finger.
Now imagine the rope is made one metre longer and lifted off the surface until it is taut.
What will its height be above the surface of the planet?
(For the purposes of the puzzle, assume that the planet is a perfect sphere).