Q.15. A and B together can complete a piece of work in 4 days. If A alone can complete the same work in 12 days, in how many days can B alone complete that work ?
Solution
(A+B)’s 1 days’s work = 1/4, A’s 1 day’s work = 1/12.
⸫ B’s 1 day’s work = [(1/4) – (1/12)]= 1/6.
Hence, b alone can complete the work in 6 days.
Q.16. Two pipes can fill a tank in 10 hours and 12 hours respectively while a third pipe empties the full tank in 20 hours. If all the three pipes operate simultaneously, in how much time will the tank be filled?
Solution
Net part filled in 1 hours = [(1/10)+(1/12)-(1/20)] = 8/60 = 2/15.
⸫ The tank will be full in 15/2 hrs = 7 hrs 30 min.
Q.17. How many minutes does Aditys take to cover a distance of 400 m, if the runs at a speed of 20 km/ hr ?
Solution
Aditya’s speed = 20 km/ hr = [20*(5/18)] m/sec = 50/9 m/sec
⸫ Time taken to cover 400 m = [ (400*(9/50)] sec = 72 sec =
Q.18. A train 100 m long is running at the speed of 30 km/hr. Find the time taken by it pass a man standing near the railway line.
Solution
Speed of the train = [30*(5/18)] m/sec = (25/3) m/sec.
Distance moved in passing the standing man = 100 m.
Required time taken = [100/(25/3)] = [100*(3/25)] sec = 12 sec.
Q.19. There is a road beside a river. Two friends started form a place A, moved to a temple situated at another place B and then returned to A again. One of them moves on a cycle at a speed of 12 km/hr, while the other sails on a boat at a speed of 10 km/hr. if the river flows at the speed of 4 km/hr, which of the two friends will. Return to place A first ?
Solution
Clearly, the cyclist moves both ways at a speed of 12 km/hr.
So, average speed of the cyclist = 12 km/hr.
The boat sailor moves downstream@ (10+4) i.e, 14 km/hr and upstream @ (10-4) i.e, 6 km/hr.
So, average speed of the boat sailor = [(2*14*6)/(14+6)] km/hr = 42/5 km/hr = 8.4/hr.
Since the average speed of the cyclist is greater, he will return to A first.
Q.20. Find the simple interest on Rs. 68,000 at 16 2/3% per annuum for 9 months.
Solution
P = Rs. 68000, R =
⸫ S.I. = [(P*R*T)/100] = Rs. [(68000*(50/3)*(3/4)*(1/100)] = Rs 37.50.
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