no intermediate stops

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Tue Mar 03, 2009 4:32 am

A train travels at the rate of 10 miles/hr for the first hour of a trip, at 20 miles/hr for the second hour, at 30 miles/hr for the third hour and so on. How many hours will it take the train to complete a 450-mile journey? Assume that the train makes no intermediate stops.

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Source: Beat The GMAT — Problem Solving | Tue Mar 03, 2009 4:32 am

praxis: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Thu Apr 03, 2008 1:20 pm; Thanked: 6 times

by praxis » Tue Mar 03, 2009 10:13 am

Each hour the train 10h, where h is the hour number.
hour 1: 10 hour 4: 100 hour 7: 280
hour 2: 30 hour 5: 150 hour 8: 360
hour 3: 60 hour 6: 210 hour 9: 450

It will take 9 hours. You can't simply use an integral, because the speed does not increase smoothly.

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