Mathematics
NULL
Medium
Question
A and B fill a cistern in 20 and 30 minutes; C empties it in 40 minutes. How long to fill if all three are opened together?
A
19 1/7 min
B
15 1/7 min
C
17 1/7 min
✓ Official correct answer
D
7 1/7 min
Community credibility
Registered members can vote for the answer they believe is correct today and explain why. If the facts change after an election or news event, suggest a new option so others can vote it up.
A
19 1/7 min
No member explanations yet for this option.
B
15 1/7 min
No member explanations yet for this option.
C
17 1/7 min
No member explanations yet for this option.
D
7 1/7 min
No member explanations yet for this option.
Explanation
Net rate = 1/20 + 1/30 − 1/40 = 7/120. Time = 120/7 = 17 1/7 min.