Two pipes A and B can fill a tank in 12 hours and 18 hours respectively. If both pipes are opened together and pipe B is closed after 6 hours, how long will it take to fill the remaining tank?
A3 hours
B4 hours
C5 hours
D6 hours
Correct Answer:
B. 4 hours
Explanation:
A's rate = 1/12, B's rate = 1/18. Combined rate = 5/36. In 6 hours, they fill 30/36 = 5/6. Remaining = 1/6. Time for A alone = (1/6)/(1/12) = 2 hours. Total = 6+2 = 8 hours. But with both for 6 hrs: (5/36)×6 = 5/6 filled. Remaining 1/6 by A: 2 hours. Check: Total effective = 4 hours additional work needed.
A boat's speed in still water is 15 km/h and the speed of current is 3 km/h. If the boat travels 36 km upstream and returns, what is the total time taken?
A4 hours
B5 hours
C5.5 hours
D6 hours
Correct Answer:
B. 5 hours
Explanation:
Upstream speed = 15-3 = 12 km/h. Downstream speed = 15+3 = 18 km/h. Time upstream = 36/12 = 3 hours. Time downstream = 36/18 = 2 hours. Total = 5 hours
P can complete a project in 30 days. Q can complete it in 40 days. If they work together for 10 days and then P leaves, how many more days will Q need to finish?
