In problems like this, where different things are combined, it is the rates that add. "3 pipes can fill a reservoir in 10, 15, 20 hours." so the rates at which the pipes fill the reservoir are 1/10, 1/15, and 1/20 "reservior per hour" respectively. Let the time gap you are looking for be "T" hours. Then you use the first pipe only for "T" hours, the first two pipes for an additional "T" hours, and all three for "5- 2T" hours. How much of reservoir will be filled in 5 hours? Set that equal to 1 and solve for T.