I think this is the problem you are referring to, as given as an example of a typical math problem.
A and B working to together can fill a cistern in 4 hours.
It takes A and C 5 hours.
B can fill the cistern twice as fast as C.
How long does it take C to fill the cistern?

Let a, b and c be the rates at which A, B and C fill the cistern. It is convenient to work with rates rather than time, because the rates can be added.
a + b =1/4
a + c = 1/5
b=2c.
Substitute 2c for b in the first equation. That gives two equations in a and c, which I trust you can easily solve. We end up with c=.05, a=.15 and b= .1.
c=.05=1/20, so it takes C 20 hours to fill the cistern.