Distance

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Fri Dec 03, 2010 4:59 am

Walking at 4/5 of his usual speed, a man is 10 minutes too late. find his usual time.

A. 81 minutes

B. 64 minutes

C. 52 minutes

D. 40 miuutes

E. none

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Source: Beat The GMAT — Problem Solving | Fri Dec 03, 2010 4:59 am

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Fri Dec 03, 2010 5:14 am

N:Dure wrote:Walking at 4/5 of his usual speed, a man is 10 minutes too late. find his usual time.
A. 81 minutes
B. 64 minutes
C. 52 minutes
D. 40 minutes
E. none

By walking at 4/5 of usual speed he will take 5/4 of his usual time.

Extra time = (5/4) of usual time - usual time = 10 minutes
=> (1/4) of usual time = 10 minute
=> usual time = 40 minutes

The correct answer is D.

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Fri Dec 03, 2010 5:20 am

Thanks Rahul! But I don't get this part: "By walking at 4/5 of usual speed he will take 5/4 of his usual time"

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gmatusa2010: Master | Next Rank: 500 Posts; Posts: 134; Joined: Sun Jul 25, 2010 5:22 amFollowed by: 2 members; Thanked: 1 times

by gmatusa2010 » Fri Dec 03, 2010 5:26 am

RT=D

4/5R(T)=1, so T has to be 5/4.

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Fri Dec 03, 2010 5:30 am

Why D=1?

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gmatusa2010: Master | Next Rank: 500 Posts; Posts: 134; Joined: Sun Jul 25, 2010 5:22 amFollowed by: 2 members; Thanked: 1 times

by gmatusa2010 » Fri Dec 03, 2010 6:16 am

1 is 100%, as in completing the job. IF at R rate and T time you can complete 1 job then RT=1; 4/5R(T)=1? if T is the the same then you can only complet 4/5 the job or 4/5R(T)=1(4/5) right?

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Fri Dec 03, 2010 9:00 am

N:Dure wrote:Walking at 4/5 of his usual speed, a man is 10 minutes too late. find his usual time.

A. 81 minutes

B. 64 minutes

C. 52 minutes

D. 40 miuutes

E. none

We can plug in the answer choices, which represent the normal time. Since the problem includes 4/5, the correct answer is likely to be a multiple of 4 and 5. Let's try answer choice D.

Answer choice D: Normal time = 40 minutes.
Let distance = 200 miles.
Rate = 200/40 = 5 miles/minute.
4/5 rate = 4/5*5 = 4 miles/minute.
Time at slower rate = 200/4 = 50 minutes.
50-40 = 10 minute increase. Success!

The correct answer is D.

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