Add a very plan a milk tank and be filled in 8 hours using the end belt using the out valve the tank can be emptied in 12 hours. if both valves are open how long will it take to fill the tank?

The general form of the quetion of a circle is ax^2+by^2+cx+dy+e=0 where a=b=0 if the circle has a radius of three units and the center lies on the y axis which set of values of a, b, c, d, and e might correspond to the circle