text: These are just similar ratios:\n\n500 gallons / 20 minutes = x gallons / 30 minutes\n\n500 / 20 = x / 30\n\nNow solve for x (you can cross multiply if you like):\n20x = 15000\nx = 750 gallons.\n\nIt's also rather intuitive to see that you got 500 gallons in 20 minutes.  So in 10 minutes more, you will pump half again as much.  500 + 250 = 750 gallons.