Atank can be filled using pipes a, b or both. it takes pipe a, running alone 18 hours to feel the tank. it takes both pipes running together 9.9 hours to fill the tank. how long does it take pipe b, running alone, to fill the tank?

For this case, what we can do is calculate an average of the time it takes to fill the tank for both pipes.
 We have then:
 taver = (tA + tB) / (2)
 Where,
 taver: average time (both pipes)
 tA: time it takes the pipe A
 tB: time taken by pipe B
 Substituting the values:
 9.9 = (18 + tB) / (2)
 Clear tB:
 tB = (2 * (9.9)) - 18
 tB = 1.8 hours
 
 it takes pipe B, running alone, to fill the tank about 1.8 hours