Pipes & Cistern:

1. Q: A pipe fills a tank in 6 hours. How much part of the tank is filled in 1 hour?
   Answer: 1/6 

2. Q: A pipe fills a tank in 8 hours. Another pipe fills it in 12 hours. Together they fill in?
   Answer: 1/8+1/12=5/24 → 4.8 hours

3. Q: A pipe fills a tank in 10 hours. Another empties it in 15 hours. Together they fill in?
   Answer: 1/10-1/15=1/30 → 30 hours

4. Q: A pipe fills a tank in 5 hours. How much part is filled in 2 hours?
   Answer: 2/5

5. Q: A pipe fills a tank in 12 hours. Another fills in 6 hours. Together they fill in?
   Answer: 1/12+1/6=1/4 → 4 hours

6. Q: A pipe fills a tank in 4 hours. Another empties it in 6 hours. Together they fill in?
   Answer: 1/4-1/6=1/12 → 12 hours 

7. Q: A pipe fills a tank in 9 hours. Another fills in 18 hours. Together they fill in?
   Answer: 1/9+1/18=1/6 → 6 hours

8. Q: A pipe fills a tank in 15 hours. Another empties it in 20 hours. Together they fill in?
   Answer: 1/15-1/20=1/60 → 60 hours

9. Q: A pipe fills a tank in 8 hours. Another empties it in 12 hours. Together they fill in?
   Answer: 1/8-1/12=1/24 → 24 hours

10. Q: A pipe fills a tank in 10 hours. Another fills in 20 hours. Together they fill in?
    Answer: 1/10+1/20=3/20 → 6 hours 40 minutes

11. Q: A pipe fills a tank in 5 hours. Another empties it in 10 hours. Together they fill in?
    Answer: 1/5-1/10=1/10 → 10 hours 

12. Q: A pipe fills a tank in 6 hours. Another fills in 3 hours. Together they fill in?
    Answer: 1/6+1/3=1/2 → 2 hours

13. Q: A pipe fills a tank in 12 hours. Another empties it in 18 hours. Together they fill in?
    Answer: 1/12-1/18=1/36 → 36 hours

14. Q: A pipe fills a tank in 7 hours. Another fills in 14 hours. Together they fill in?
    Answer: 1/7+1/14=3/14 → 4 hours 40 minutes

15. Q: A pipe fills a tank in 4 hours. Another empties it in 8 hours. Together they fill in?
    Answer: 1/4-1/8=1/8 → 8 hours

16. Q: A pipe fills a tank in 10 hours. Another empties it in 5 hours. Together they fill in?
    Answer: 1/10-1/5=-1/10 → Tank never fills (empties in 10 hours). 

17. Q: A pipe fills a tank in 12 hours. Another fills in 15 hours. Together they fill in?
    Answer: 1/12+1/15=9/60=3/20 → 6 hours 40 minutes

18. Q: A pipe fills a tank in 18 hours. Another empties it in 27 hours. Together they fill in?
    Answer: 1/18-1/27=1/54 → 54 hours

19. Q: A pipe fills a tank in 24 hours. Another fills in 30 hours. Together they fill in?
    Answer: 1/24+1/30=9/120=3/40 → 13 hours 20 minutes

20. Q: A pipe fills a tank in 8 hours. Another empties it in 16 hours. Together they fill in?
    Answer: 1/8-1/16=1/16 → 16 hours