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rate prob gmat

by quantskillsgmat » Tue Dec 27, 2011 10:28 pm
The reduction in speed of a railway engine is directly proportional to square root of number of compartment attached to it. with no compartments attached speed is 42m/hr. with 9 compartments speed is 24m/hr.what are maximum number of compartments that can be attached to the engine.
a)49
b)48
c)46
d)47
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by shankar.ashwin » Wed Dec 28, 2011 2:54 am
Original speed = 42

Reduction in speed = k * Sqrt(No. of compartments)

Given speed = 24 for 9 compartments.

(42-24) = k * Sqrt(9)

18 = 3k

k = 6

The train will stop when 'Reduction in speed = 42'

So, 42 = 6 * Sqrt(No of compartments)

Sqrt(No of compartments) = 7

No. of compartments = 49.

Since the train will be stationary when 49 compartments are attached, 48 is the maximum that can be attached to keep it moving. B
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