Hello Alchemist9When viewed from the front (in the drawing given) the cross-section of the water (when it's 6" from the top) is a trapezium* (trapezoid) whose parallel sides are 14' and 36' in length, and height is 10'; plus a rectangle whose length is 50', height 3.5'.

So, work out the total area; multiply it by the width of the pool (20') and there's the volume, in cubic feet, of water required. Multiply by 7.5 to convert to gallons and divide by 10 to get the number of minutes it will take (because the rate of flow is 10 gallons per minute).

OK now?

Grandad

(* Area of trapezium = half the sum of the parallel sides times the perpendicular distance between them.)