A water vendor has a tank of capacity18,900 litres. The tank is being filled with water from two pipes A and B which are closed immediately when the tank is full. Water flows at the rate 150,000cm^{3}/minute through pipe B.

- If the tank is empty ,and the two pipes are opened at the same time, calculate the time it takes to fill the tank.
- On a certain day the vendor opened the two pipes A and B to fill the empty tank. After 25 minutes he opened the outlet tap to supply water to his customer at average rate of 20 litres per minute. Calculate the time it took to fill the tank on that day.