Which Tank Fills?
Read the puzzle
A tap pours into a top tank with two side pipes: a low left pipe, sealed by a red cap, leading to tank A, and a higher right pipe leading to tank B. Tank A has a low left pipe to tank C and a higher right pipe to tank D. Tank B has a low left pipe to tank D and a higher right pipe, near its rim, to tank F. Tank D has one pipe, just below its rim, to tank E. Tanks C, E and F have no outlets. Which tank fills up first?
Which tank fills up FIRST?
Water leaves a tank by its lowest open pipe.
E
The cap stops A. Water takes each lowest open pipe: B, then D, then E.
How to solve it
Follow the water one tank at a time and always send it out through the lowest open pipe, because water leaves a tank as soon as it reaches its first outlet.
The top tank has two pipes. The lower one, on the left, leads to tank A but is sealed by the red cap, so nothing goes that way. The water rises to the higher right pipe and drops into tank B.
Tank B has a pipe on each side. The left pipe, to tank D, sits lower than the right pipe to tank F, so B only fills up to its left pipe and spills into D. F never gets a drop.
Tank D has a single pipe, on its right, running into tank E. Once D reaches that pipe, everything flows on to E. E has no outlet at all, so it is the tank that fills up completely first.
The common wrong answer is A, because its pipe is the lowest on the top tank, but the cap blocks it. Others pick B or D, which only fill as high as their outlets. Quick check: the first tank to fill is always one with no open way out.
