Below are three simple and straightforward problems from boats and streams. You can expect these kinds of questions and their variants on placement papers of top companies. Most of the times, these questions tend to be very easy and if you are capable of solving these questions quickly, you are prone to get significant time advantage over the others.
A boat covers a distance of 30 kms in 2 ½ hours running down streams. While returning, it covers the same distance in 3 ¾ hours. What is the speed of the boat in kmph?
a) 12 b)8 c) 14 d) 10
Answer : d) 10
Let the speed of boat in still water be B km per hour
Let the speed of stream be S km per hour
Downstream speed of the boat = B + S = 30 divided by 2 ½ = 12 km/hours ....(I)
Upstream speed of the boat = B – S = 30 / 3 ¾ = 8 km/hour .....(II)
Solving (I) and (II) we get B = 10 km per hour.
A boat travels 72 km downstream in 8 hours and 84 km upstream in 12 hours. Find the speed of the boat in still water and the speed of the water current.
a) 8 km/hr, 1 km/hr b)5 km/hr, 4 km/hr c) 1 km/hr, 6 km/hr d)2 km/hr, 1 km/hr
Answer : a)8 km/hr, 1 km/hr
Let D be the downstream speed, let U be the upstream speed, let B be the still water speed of the boat and let C be the water current speed.
D = 72/8 = 9 km/hr
U = 84/12 = 7 km/hr
Knowing D and U, B can be obtained by the formula, B =( D+U ) / 2 = (9 + 7) / 2 = 16/2 = 8 km/hr
C = 9-8 = 1 km/hr
A boat can row 18 kmph in still water.It takes twice as long to row up as to row down the river.Find the rate of stream.
a)6 km/hr b)7 km/hr c) 8km/hr d)5 km/hr
Answer : a)6 km/hr
(This question can be answered by remembering two important but very simple formulas.)
Let boat's upstream speed be x km/hr. Then, the downstream speed is 2x km/hr.
Speed of the boat in still water = 1/2(speed of downstream + speed of upstream)
i.e. 18 = 1/2 (2x + x)
x = 12
Speed of upstream is 12 km/hr & the speed of downstream is twice the upstream so 24 km/hr.
Rate of stream = 1/2 (speed of downstream - speed of upstream)
=1/2 (24 - 12)
= 6 km/hr
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