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Mark biked from his house to his friend's house in how many [#permalink]

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25 Oct 2012, 20:44

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Mark biked from his house to his friend's house in how many hours?

(1) Mark bikes at an average speed of 72 blocks per hour.

(2) If Mark could bike an extra 8 blocks for each hour, he would have arrived at his friend's place 1 hour earlier

I got the correct answer (no spoilers!) but if I were to sit down and calculate the answer, how would I do it (I know purpose is not to calculate in DS, but it's really for practice, if anything).

Mark biked from his house to his friend's house in how many hours?

1. Mark bikes at an average speed of 72 blocks per hour.

2. If Mark could bike an extra 8 blocks for each hour, he would have arrived at his friend's place 1 hour earlier

I got the correct answer (no spoilers!) but if I were to sit down and calculate the answer, how would I do it (I know purpose is not to calculate in DS, but it's really for practice, if anything).

Thanks again.

Mark bikes at an average speed of 72 blocks per hour. Mark could bike an extra 8 blocks for each hour = 80 blocks per hour

LCM of 72 & 80 = 720. Time taken by Mark = 720/72 = 10 hrs.

Cheers!
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----------------------------------------------------------------------------------------- What you do TODAY is important because you're exchanging a day of your life for it! -----------------------------------------------------------------------------------------

In all cases, if the speed increases by 1/x, the time taken decreases by 1/x+1 (as speed and time taken are inversely proportional to each other).

In this case the increase in speed is 1/9 (Mark's new speed is 72+8=80 blocks per hour, and the increase of 8 blocks per hour is 1/9th of his previous speed).

Therefore the corresponding decrease in time = 1/9+1 = 1/10. This represents 1/10th of the actual time taken.

1/10th of the actual time taken is given as 1 hour. Hence the total time taken is 10 hours.

Re: Mark biked from his house to his friend's house in how many [#permalink]

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01 Aug 2013, 13:08

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This post received KUDOS

Mark biked from his house to his friend's house in how many hours?

Time = Distance/Speed

(1) Mark bikes at an average speed of 72 blocks per hour. There is no information given about the distance Mark has to travel. All we know is that r=72 INSUFFICIENT

(2) If Mark could bike an extra 8 blocks for each hour, he would have arrived at his friend's place 1 hour earlier t-1 = d/(r+8) We can try and plug in various other distance/time/rate formulas to try and cancel out variables but it's unlikely that will leave us with only one given that there are three variables to plug in for. INSUFFICIENT

1+2) r=72 and t-1 = d/(r+8) We can plug in for r but we still have variables t and d left. d=r*t. If we were to substitute for d we could cancel out t leaving us with just t, as we are looking for the time it took him to bike to his friends house. t-1 = d/(r+8) t-1 = r*t/(72+8) t-1 = 72*t/(80) 80(t-1) = 72t 80t-80=72t 8t=80 t=10 SUFFICIENT

Re: Mark biked from his house to his friend's house in how many [#permalink]

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24 Oct 2013, 21:41

WholeLottaLove wrote:

Mark biked from his house to his friend's house in how many hours?

Time = Distance/Speed

(1) Mark bikes at an average speed of 72 blocks per hour. There is no information given about the distance Mark has to travel. All we know is that r=72 INSUFFICIENT

(2) If Mark could bike an extra 8 blocks for each hour, he would have arrived at his friend's place 1 hour earlier t-1 = d/(r+8) We can try and plug in various other distance/time/rate formulas to try and cancel out variables but it's unlikely that will leave us with only one given that there are three variables to plug in for. INSUFFICIENT

1+2) r=72 and t-1 = d/(r+8) We can plug in for r but we still have variables t and d left. d=r*t. If we were to substitute for d we could cancel out t leaving us with just t, as we are looking for the time it took him to bike to his friends house. t-1 = d/(r+8) t-1 = r*t/(72+8) t-1 = 72*t/(80) 80(t-1) = 72t 80t-80=72t 8t=80 t=10 SUFFICIENT

(C)

if we take only second statement (x+8)(t-1)=distance to his friends home=xt.................cant we find t from here ? what am i doing wrong

Mark biked from his house to his friend's house in how many hours?

Time = Distance/Speed

(1) Mark bikes at an average speed of 72 blocks per hour. There is no information given about the distance Mark has to travel. All we know is that r=72 INSUFFICIENT

(2) If Mark could bike an extra 8 blocks for each hour, he would have arrived at his friend's place 1 hour earlier t-1 = d/(r+8) We can try and plug in various other distance/time/rate formulas to try and cancel out variables but it's unlikely that will leave us with only one given that there are three variables to plug in for. INSUFFICIENT

1+2) r=72 and t-1 = d/(r+8) We can plug in for r but we still have variables t and d left. d=r*t. If we were to substitute for d we could cancel out t leaving us with just t, as we are looking for the time it took him to bike to his friends house. t-1 = d/(r+8) t-1 = r*t/(72+8) t-1 = 72*t/(80) 80(t-1) = 72t 80t-80=72t 8t=80 t=10 SUFFICIENT

(C)

if we take only second statement (x+8)(t-1)=distance to his friends home=xt.................cant we find t from here ? what am i doing wrong

Please try to solve and you'll get the answer yourself.
_________________

Re: Mark biked from his house to his friend's house in how many [#permalink]

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15 Jan 2015, 13:50

Hello from the GMAT Club BumpBot!

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Re: Mark biked from his house to his friend's house in how many [#permalink]

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26 Sep 2016, 18:37

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
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Mark biked from his house to his friend's house in how many [#permalink]

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16 Oct 2016, 09:10

Let's assume that distance between the two houses is x blocks.

(1) With only avg. speed given, S = 72 blocks per hour, and no other information, it not possible to find the time. One equation, two variables \(t = \frac{x}{72}\) INSUFFICIENT

(2) Given \(\frac{x}{S} = \frac{x}{(S+8)} + 1\), equation with two variables and no other constraints on x or/and S, it can not be solved. INSUFFICIENT

However, combining (1) and (2), we can see that substituting value of S in (2) will give us x and then we can get time using \frac{x}{S}. (We don't even need to calculate the exact values.) Hence, (1) and (2) together are SUFFICIENT.

C

gmatclubot

Mark biked from his house to his friend's house in how many
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16 Oct 2016, 09:10

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