Problem 4. A basin is filled with one pipe. If the water flow into the basin is reduced by $20 \%$, by what percentage will the filling time of the basin increase?
Solution. Let $a$ liters of water flow out of the pipe into the tank in 1 hour, and let the tank be filled for $x$ hours. Then the volume of the tank is $V=a x$ liters. If the water flow is reduced by $20 \%$, then $0.8 a$ liters will flow into the tank per hour. If $y$ is the time required to fill the tank in this case, then for the volume of the tank we have $V=0.8 a y$. Therefore, $a x=0.8 a y$, from which we get $10 x=8 y$, i.e.

$$
y=1.25 x=x+0.25 x
$$

which means that with a $20 \%$ reduction in water flow, the time to fill the tank will increase by $25 \%$.