A pipe can fill a tank in 15 hours. Another pipe can empty it in 20 hours. If both pipes are opened together, in how many hours will the tank be filled?
A45 hours
B50 hours
C60 hours
D75 hours
Correct Answer:
C. 60 hours
Explanation:
Filling rate = 1/15 per hour. Emptying rate = 1/20 per hour. Net rate = 1/15 - 1/20 = (4-3)/60 = 1/60. Time = 60 hours.
A and B together can complete a work in 12 days. A alone can complete it in 20 days. In how many days can B alone complete the work?
A25 days
B30 days
C35 days
D40 days
Correct Answer:
B. 30 days
Explanation:
Work done by A in 1 day = 1/20. Work done by A+B in 1 day = 1/12. So B's work in 1 day = 1/12 - 1/20 = (5-3)/60 = 2/60 = 1/30. Therefore B takes 30 days.
A contractor agrees to build a bridge in 300 days. He employs 10 workers. After 150 days, he finds that only half the work is complete. How many additional workers does he need to finish on time?
A5 workers
B10 workers
C15 workers
D20 workers
Correct Answer:
B. 10 workers
Explanation:
Remaining days = 150. Remaining work = 1/2. Current productivity = (1/2 work)/(150 days × 10 workers) = 1/3000 per worker-day. Required rate = (1/2)/(150 × x) where x is total workers. x = 10. So need 10 additional workers.
Worker A takes 18 days to complete a job. Worker B takes 12 days. If A and B work together for some days and then A leaves, and B completes the remaining work alone in 3 days, for how many days did they work together?
A4 days
B5 days
C6 days
D7 days
Correct Answer:
B. 5 days
Explanation:
A's rate = 1/18, B's rate = 1/12. B alone for 3 days = 3/12 = 1/4. Remaining = 3/4. Combined rate = 1/18 + 1/12 = 5/36. Time together = (3/4)/(5/36) = 27/5 = 5.4 ≈ 5 days
