The volume of a tank is 72 cubic metres. Water is poured into it at the rate of 60 litres per minute. How much time will it take to fill the tank?

Three pipes A, B and C can fill a tank from empty to full in 30 minutes, 20 minutes, and 10 minutes respectively. When the tank is empty, all the three pipes are opened. A, B and C discharge chemical solutions P, Q and R respectively. What is the proportion of the solution R in the liquid in the tank after 3 minutes?

A. $$\frac{5}{{11}}$$

B. $$\frac{6}{{11}}$$

C. $$\frac{7}{{11}}$$

D. $$\frac{8}{{11}}$$

Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in:

A. $$1\frac{{13}}{{17}}$$ hours

B. $$2\frac{8}{{11}}$$ hours

C. $$3\frac{9}{{17}}$$ hours

D. $$4\frac{1}{2}$$ hours

A pump can fill a tank with water in 2 hours. Because of a leak, it took $$2\frac{1}{3}$$ hours to fill the tank. The leak can drain all the water of the tank in:

A. $$4\frac{1}{3}$$ hours

B. 7 hours

C. 8 hours

D. 14 hours

Two pipes A and B can fill a cistern in $$37\frac{1}{2}$$ minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the B is turned off after:

A. 5 min.

B. 9 min.

C. 10 min.

D. 15 min.