How many times do you have to cut a piece of A4 before it's the width of an atom?

Q. How many times would you have to cut the long side of a piece of A4 paper in half before it's the width of an atom?

31?

310?

3100?

This question appears on overhead advertising boards on the DART (Dublin local commuter train). There’s a mobile number so you can text the Institute of Physics in Ireland to check if your answer is right.

Answer: 31

Click for other teasers, such as what happens if you try to light a candle in an orbiting space station.