coal burn trip:-)

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Shamhoon: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Sun Aug 22, 2010 5:39 am; Location: Kingdom of Saudi Arabia; GMAT Score:320

by Shamhoon » Wed Mar 16, 2011 7:37 am

The amount of coal a train burns each mile is directly proportional to the speed at which it travels. How much coal will it burn on this particular 60 mile trip?

(1) On a previous trip, the train burned 100 pounds of coal on a 60 mile trip at 60 miles per hour.

(2) On this particular trip, the train is traveling at a speed of 30 miles per hour.

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Source: Beat The GMAT — Data Sufficiency | Wed Mar 16, 2011 7:37 am

force5: Legendary Member; Posts: 582; Joined: Tue Mar 08, 2011 12:48 amFollowed by: 6 members; Thanked: 61 times; GMAT Score:740

by force5 » Wed Mar 16, 2011 9:26 am

The amount of coal a train burns each mile is directly proportional to the speed at which it travels. How much coal will it burn on this particular 60 mile trip?

(1) On a previous trip, the train burned 100 pounds of coal on a 60 mile trip at 60 miles per hour.

(2) On this particular trip, the train is traveling at a speed of 30 miles per hour.

Good question? what is the source?

here is the answer
IMO- C

we know amt of coal is directly proportional to speed. now this only tells us that amt of coal increases as speed increases.
we need to find how much coal will burn in 60 mile trip. so we need time taken. and relation between how much coal is consumed and speed.
(1) stmnt 1 gives us the relation between consumption of coal and speed (insufficient)

(2) stmnt 2 gives us time 2 hours. (insufficient)

combining
we have both the requirements hence can calculate the amount of coal needed.

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Geva@EconomistGMAT: GMAT Instructor; Posts: 905; Joined: Sun Sep 12, 2010 1:38 amFollowed by: 123 members; Thanked: 378 times; GMAT Score:760

by Geva@EconomistGMAT » Thu Mar 17, 2011 6:34 am

Answer is indeed C.

directly proportional means "whatever you multiply/divide one, do the same for the other". If, as stat. (2) tells us, the train travels on this trip at half the speed of the previous trip, it will burn half the coal of the previous trip (divide by 2 on both ends); We just need to know how much the train burned on the previous trip, which is given in stat. (1).

Geva
Senior Instructor
Master GMAT
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