Please take a look at this PS Speed-Time problem.

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aman88: Master | Next Rank: 500 Posts; Posts: 142; Joined: Sat Oct 20, 2012 3:31 am; Location: New Delhi; Thanked: 20 times; Followed by:3 members

Please take a look at this PS Speed-Time problem.

by aman88 » Wed Dec 12, 2012 2:42 am

Q: Yesterday it took a certain plane 3 hours to fly from City A to City B at an average speed of 400 miles per hour. Today the same plane flew from City A to City B along the same route at an average speed of 450 miles per hour. How many more yesterday than it took today?

A. 10
B. 15
C. 20
D. 25
E. 30

I reached till this point:
1200/450 = 8/3
OA C
Can someone please let me know how to proceed ahead. I am sure I am missing onto something here.

Thanks.

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Source: Beat The GMAT — Problem Solving | Wed Dec 12, 2012 2:42 am

eaakbari: Master | Next Rank: 500 Posts; Posts: 435; Joined: Mon Mar 15, 2010 6:15 am; Thanked: 32 times; Followed by:1 members

by eaakbari » Wed Dec 12, 2012 6:45 am

I prefer making a quick table in DST questions however easy the question. It helps avoid lousy mistakes.

D S T
d 400 3
d 450 t

Distance is the same and & since D = S* T

400 * 3 = 450 * t

Solving for t

t = 3 *400 / 450 = 8/3

Time difference = 3-t = 3 - 8/3 = 1/3 (hours)

1/3 * 60 = 20 minutes

Hence C

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Wed Dec 12, 2012 8:07 am

aman88 wrote:Q: Yesterday it took a certain plane 3 hours to fly from City A to City B at an average speed of 400 miles per hour. Today the same plane flew from City A to City B along the same route at an average speed of 450 miles per hour. How many more yesterday than it took today?

A. 10
B. 15
C. 20
D. 25
E. 30

You're missing part of the question. Presumably, we want to find out how many minutes more the first trip was.

First find the distance from A to B
Distance = (rate)(time)
= (400)(3)
= 1200 miles

Now find the time for the second trip.
Time = (distance)/(rate)
= 1200/450
= 8/3 hours
= 2 2/3 hours

So, the first trip took 3 hours, and the second trip took 2 2/3 hours.
So, the first trip took 1/3 hours longer.
1/3 hours = [spoiler]20 minutes = C[/spoiler]

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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puneetkhurana2000: Master | Next Rank: 500 Posts; Posts: 131; Joined: Wed Nov 14, 2012 2:01 pm; Thanked: 39 times; Followed by:2 members

by puneetkhurana2000 » Wed Dec 12, 2012 9:17 am

Distance between City A and City B will be same in both the cases, so

Distance in Case 1 = Distance in Case 2

400*3 = 450*t, solving for t we get t = 8/3

Difference in times = 3-(8/3) = 1/3 of an hour i.e. 20 minutes.

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

by GMATGuruNY » Wed Dec 12, 2012 12:10 pm

aman88 wrote:Q: Yesterday it took a certain plane 3 hours to fly from City A to City B at an average speed of 400 miles per hour. Today the same plane flew from City A to City B along the same route at an average speed of 450 miles per hour. How many more yesterday than it took today?

A. 10
B. 15
C. 20
D. 25
E. 30

Faster rate : slower rate = 450/400 = 9/8.
Since rate and time are RECIPROCALS, faster time : slower time = 8/9.
Since the faster time is 8/9 of the slower time, the faster time requires 1/9 fewer minutes than the slower time (180 minutes):
(1/9)180 = 20.

The correct answer is C.

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aman88: Master | Next Rank: 500 Posts; Posts: 142; Joined: Sat Oct 20, 2012 3:31 am; Location: New Delhi; Thanked: 20 times; Followed by:3 members