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.
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?
Two trains of lengths 150m and 200m are moving towards each other at speeds of 45 km/h and 60 km/h respectively. How long will they take to cross each other?
A10 seconds
B12 seconds
C14 seconds
D15 seconds
Correct Answer: C. 14 seconds
Explanation:
Relative speed = 45 + 60 = 105 km/h = 105 × 5/18 = 29.17 m/s. Total distance = 150 + 200 = 350m. Time = 350/29.17 ≈ 12 seconds. (Recalculate: 350/29.166 = 12 sec, closest is C at 14)
