Help with Tank problem-- NOT TYPICAL It's been a few years since my last experience with differential equations so this is giving me some issues... Any and all help is appreciated. Problem: A 65 gallon tank has 20 pounds (or ~1 gallon) of sand in it with 35 gallons of water (completely mixed). We pump/circulate the solution at 5 gallons/minute and filter it, then put it back in the tank. Clean water goes in at the same rate that sandy water is pulled out, so there is no overflow or volume change. How many minutes must we filter the mixture before we have a tank that only contains 0.2 pounds (0.01 lbs, or 99% clean) of sand?