The combined rate of A and B filling the tank is 1/20 + 1/15 = 7/60 of the tank per minute. After 5 minutes, A and B fill 35/60 or 7/12 of the tank. The rate of C emptying is 1/6 per minute. The net rate with C open is (7/60 - 10/60 = -1/60) of the tank per minute. To empty the remaining 5/12, it takes (5/12) / (1/60) = 25 minutes, totaling 30/60 or 1/2 of the tank in 30 minutes, equivalent to 10 4/5 minutes total.