There is a formula: (Pump1 Time * Pump2 Time) / (Pump1 Time + Pump2 Time) = Total Time Let's say Pump 1 is the old pump (15 * Pump2 Time) / (15 + Pump2 Time) = 5 Hours Solving for Pump 2 time Pump 2 time = (Pump 1 time * Total Time) / (Pump 1 time - Total Time) Pump 2 time = (15 * 5) / (15 - 5) Pump2 time = 75 / 10 Pump2 time = 7.5 Hours and Happy New Year!!!!!
Double Check (15 * 7.5) / (15 + 7.5) = Total Time 112.5 / 22.5 = 5 Hours Total