Together,two pipes can fill a reservoir in 3/4 of an hour.Pipe1 needs one hour ten minutes(1 1/6 hours)to fill the reservoir by itself.How long would pipe 2 need to fill the reservoir by itself?
Hard question. Wait until grandpa wakes up! @CPhill, @Melody, @Alan, @hugo aka sir grapes, and @Omi67 the sheep, u gotta solve this for our precious guest. 
+33615 Let r1 = fill rate of pipe 1, r2 = fill rate of pipe 2, V = reservoir volume.
r1 = V/(7/6) = 6V/7 (1)
r2 = V/t (2) t = time for pipe 2 to fill reservoir by itself
r1 + r2 = V/(3/4) = 4V/3 (3)
Use (1) in (3)
6V/7 + r2 = 4V/3 so r2 = 10V/21 (4)
Use (4) in (2)
10V/21 = V/t t = 21/10 t = 2hrs 6mins
