One water pump can fill half of a certain empty tank in 3 hours. Another pump can fill half of the same tank in 312312 hours. Working together, how long will it take these two pumps to fill the entire tank? Choose the correct choice from the following choices:	A) 1 7/13	B) 1 5/8	C) 3 1/4	D) 3 3/13	E) 3 1/2
One pump can fill a tank in 3 hours and another in 3.5 hours   so the rate at which both can half fill the tank is (1/3+1/3.5) =&gt; 13/21   Thus half of the tank can be filled in 21/13   so for filling the complete tank =&gt; 21/13*2 = 42/13   ANSWER:3 3/13
<result>D</result>
One water pump can fill half of a certain empty tank in 3 hours. Another pump can fill half of the same tank in 312312 hours. Working together, how long will it take these two pumps to fill the entire tank?