You have a cup of water held up in the air with 3 straws inserted into the bottom of the cup. Each straw has a different diameter but the same length. The cup is allowed to drain through these straws. The flow rate through each straw is measured as is the total time the cup takes to drain.

You are allowed to change the diameter of one straw by 10%. Which one do you change so as to make the cup drain as quickly as possible.

This is not really a puzzle about free surface flows, continuum fluid dynamics or the like.

And no, you can't just change each one in turn and observe the solution.

Good luck folks (I won't pretend I know the answer!).

The cup will drian the fastest ,if we reduce the smallest diameter by 10% .

Errrr, OK. Is that always the case? How did you arrive at the answer? Surely it is how one arrives at the answer that most benefits understanding.