Proportion and velocity

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manelgirona: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Thu Oct 15, 2009 11:35 am

by manelgirona » Mon Feb 15, 2010 3:09 pm

If the distance required for an automobile to stop varies directly as the square of its speed, what is the distance required for an automobile driven at 60 kilometers an hour to stop if it takes 10 meters to stop the automobile at 20 kilometers an hour?

a) 30 meters
b) 36 meters
c) 60 meters
d) 90 meters
e) 120 meters

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Source: Beat The GMAT — Problem Solving | Mon Feb 15, 2010 3:09 pm

djkvakin: Senior | Next Rank: 100 Posts; Posts: 84; Joined: Sat Oct 03, 2009 1:13 pm; Thanked: 11 times

by djkvakin » Mon Feb 15, 2010 6:42 pm

the square of the 20mph speed is 400 and we know that the breaking distance is 10. from here we set up a proportion:
10/400=x/3600. Solve for x
we get x=90 = D

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Mon Feb 15, 2010 6:50 pm

manelgirona wrote:If the distance required for an automobile to stop varies directly as the square of its speed, what is the distance required for an automobile driven at 60 kilometers an hour to stop if it takes 10 meters to stop the automobile at 20 kilometers an hour?

a) 30 meters
b) 36 meters
c) 60 meters
d) 90 meters
e) 120 meters

Triple the speed means 3^2 times the stopping distance.

9 * 10 = 90 metres!

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Mon Feb 15, 2010 9:25 pm

manelgirona wrote:If the distance required for an automobile to stop varies directly as the square of its speed, what is the distance required for an automobile driven at 60 kilometers an hour to stop if it takes 10 meters to stop the automobile at 20 kilometers an hour?

a) 30 meters
b) 36 meters
c) 60 meters
d) 90 meters
e) 120 meters

D = function(v^2)
so D = k V^2
10 = k (20^2)

Hence[spoiler] D= 60^2 x 10 /20^2 = 90 meters Ans D[/spoiler]

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