As all new fathers do, I ponder calculus problems when I give my son a bath. OK you wash the kid's hair, so now the bath water is soapy. There are two things you can do to try to reduce the soapiness (specifically, soap units divided by volume of water in tub). (A) You can pull the plug so the tub starts to drain. (B) You can turn the faucet on so fresh water pours into the tub.
What is the fastest way to reduce the soapiness? (A), (B), or both? (HINT: I originally convinced myself that one of the techniques was "obviously" in the answer, but then I later reversed myself. I.e. I think one of the techniques is irrelevant.)