Topper's Approach 1

We will be solving Pipe & Cistern Question in the same way we solved Work & Time question by assuming number of Pens per day Trick.
1) Two pipes A and B can fill a tank in 20 minutes and 30 minutes respectively. If both pipes are opened together, the time taken to fill the tank is:
a) 50 minutes
b) 12 minutes
c) 25 minutes
d) 15 minutes
Answer: 12 minutes
2) A tap can fill a cistern in 8 hours and another tap can empty it in 16 hours. If both the taps are open, the time taken to fill the tank will be:
a) 8 hours
b) 10 hours
c) 16 hours
d) 24 hours 
Answer: 16 hours
3) Two pipes can fill a cistern separately in 10 hours and 15 hours. They can together fill the cistern in: 
a) 6 hours
b) 7 hours
c) 8 hours
d) 9 hours
Answer: 6 hours