You just learnt the art of making the most perfect Boiled Egg...
'boiling it for exactly 15 minutes'
your clock ran out of battery,
you left your watch at sleepover,
and your phone got stolen (poor you..)
all you have are two sand-clocksa 7-minute one,
and another 11-minute one...
how can you use them both
to boil your egg for exactly 15 minutes?