Interesting GMAT Question

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showbiz: Senior | Next Rank: 100 Posts; Posts: 44; Joined: Tue Jun 22, 2010 12:15 am; Thanked: 3 times

by showbiz » Mon Jul 30, 2012 1:18 am

You get up in the morning at 5:25 Am and start solving math questions at 30q/hr. Your friend gets up at 6:15 and starts solving at 40q/hr. At what time will your friend catch up?

[spoiler]8:45[/spoiler]

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Source: Beat The GMAT — Problem Solving | Mon Jul 30, 2012 1:18 am

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 » Mon Jul 30, 2012 2:54 am

showbiz wrote:You get up in the morning at 5:25 Am and start solving math questions at 30q/hr. Your friend gets up at 6:15 and starts solving at 40q/hr. At what time will your friend catch up?

[spoiler]8:45[/spoiler]

From 5:25am to 6:15am, the number of questions answers by you in 5/6 of an hour = r*t = 30*(5/6) = 25.
The CATCH-UP rate is equal to the DIFFERENCE between the two rates:
40-30 = 10 questions per hour.
Time for your friend to catch-up by 25 questions = w/r = 25/10 = 2.5 hours.
Since your friend starts at 6:15am, the time at which your friend catches up = 6:15 + 2.5 hours = 8:45am.

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Lifetron: Master | Next Rank: 500 Posts; Posts: 308; Joined: Thu Mar 29, 2012 12:51 amFollowed by: 3 members; Thanked: 16 times

by Lifetron » Mon Jul 30, 2012 3:40 am

At 6.15, I would have completed 25 questions. I am now 25 Qs ahead of my friend.

My friend for every hour will complete my 30 questions and an other 10, which will get reduced from my initial extra of 25. So, for every hour 10 will get subtracted from 25. So, we need 1+1+0.5 = 2.5 hours for the total of 25. 6.15 am + 2.5 h = 8.45 AM

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