Question 774651
The rates are additive when both pipes are fully open at the same time.

Filling pipe, {{{1/8}}} tank per hour
Emptying pipe, {{{-1/4}}} tank per hour


Combined Rate, {{{(1/8-1/4)}}} tank per hour.  Notice very carefully that rate of filling is smaller than the rate of emptying.  YOU CANNOT FILL THIS TANK WHEN BOTH PIPES ARE OPEN!!


Simplify to {{{1/8-2/8=highlight(-1/8)}}} tank/hour.
This tank will never fill when both pipes are open all the way.