Question

With a 5-minute sand timer and a 9-minute sand timer, what is the easiest way to time an egg to boil for 13 minutes?

Answer 1

What grade is this for?

Answer 2

Start both timers.
When the 5-minute timer runs out of sand it signifies that 5 minutes has passed.
Meaning there should be 4 minutes worth of sand left in the 9 minute timer.

Drop the egg in at this point.
When the remaining 4 minutes worth of sand in 9-minute timer runs out, flip it over,
When it runs out of sand another 9 minutes will have passed.

4 + 9 = 13 minutes! :)