A water tank has two pipes to fill water in it. One of the pipes fills water at the rate of 100 litres per hour. The tank is filled by the other pipe in 6 hours and by both pipes together in 4.2 hours. What is the capacity of the tank in litres ? Choose the correct choice.
A) 860 litre
B) 980 litre
C) 1120 litre
D) 1400 litre
E) None of these
Explanation:   Let the time taken by the 1st pipe to fill the tank be x hours.   When both the pipes are open, it is given that:   1/x + 1/6 = 1/(4.2) of tank is filled in 1 hour.   1/x + 1/6 = 5/21   21(6 + x) = 30 x   9x = 126   x = 14 hours   Therefore, capacity of the tank = 100 * 14= 
<gadget id="calculator">100 * 14</gadget>
<output>1_400</output>
1400 litres.   ANSWER 1400 litre
<result>D</result>
A water tank has two pipes to fill water in it. One of the pipes fills water at the rate of 100 litres per hour. The tank is filled by the other pipe in 6 hours and by both pipes together in 4.2 hours. What is the capacity of the tank in litres ?