A water tank is filled by two pipes in 20 hours. One of the pipes fills it alone 30 hours faster than…

The slower pipe fills the tank in 60 hours. If its time is $x$ hours, the faster pipe takes $x-30$ hours, and $1/x+1/(x-30)=1/20$. This simplifies to $x^2-70x+600=0$, whose roots are 60 and 10; only 60 gives a positive time for both pipes. Thus the separate times are 60 and 30 hours, and their combined rate is 1/20 tank per hour.