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
 Answer:
 it takes pipe B, running alone, to fill the tank about 1.8 hours