1. The problem statement, all variables and given/known data Three pumps are attached to a three-dimensional tank. They pump water into the tank at a constant rate. Pump A when turned on alone fills up the tank in 15min. Pump B when turned on alone fills up the tank in 30min. Pump C when turned on alone fills up the tank in 60min. If ALL THREE pumps are turned on at the same time, how long will it take to fill up the tank? 2. Relevant equations 3. The attempt at a solution The volume of the tank doesn't matter in this question. So I tried to find the rates at which each pump filled up the tank. Pump A: Rate A = 1 tank/15min Pump B: Rate B = 1 tank/30min Pump C: Rate C = 1 tank/60min I then added the rates, since they are each constant: Rate A + Rate B + Rate C = 7tank/60min = 1 tank/8.57min. 8.57 min to fill the tank with all three pumps turned on. This was incorrect. I'm kicking myself because I've already taken classical mechanics and a sufficient amount of math courses, yet I can't seem to think out this simple problem.