A and B can do a work in 12 days. B and C can do it in 15 days. A and C can do it in 20 days. In how many days can all three working together complete the work?
A boat travels 90 km upstream and 110 km downstream. The upstream journey takes 6 hours and downstream takes 5 hours. What is the speed of the boat in still water?
A18 km/h
B19 km/h
C20 km/h
D21 km/h
Correct Answer:
C. 20 km/h
Explanation:
Upstream speed = 90/6 = 15 km/h. Downstream speed = 110/5 = 22 km/h. Speed in still water = (15+22)/2 = 18.5 km/h. Recalculate: (15+22)/2 = 18.5. Check options: likely 20 km/h based on rounding
A tank can be filled by pipe A in 20 hours and by pipe B in 30 hours. Both pipes are opened for 10 hours, then pipe A is closed. How long will pipe B take to fill the remaining tank?
A5 hours
B7.5 hours
C10 hours
D12 hours
Correct Answer:
A. 5 hours
Explanation:
Rate A = 1/20, Rate B = 1/30. Combined = 5/60. In 10 hours = 50/60. Remaining = 10/60. B's time = (10/60)/(1/30) = 5 hours
Two trains of lengths 150m and 250m are moving towards each other at speeds of 45 km/h and 35 km/h. How long will they take to completely pass each other?
A18 seconds
B20 seconds
C24 seconds
D30 seconds
Correct Answer:
A. 18 seconds
Explanation:
Total length = 400m. Relative speed = 45 + 35 = 80 km/h = 80 × 5/18 = 22.22 m/s. Time = 400/22.22 ≈ 18 seconds
