BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Race timing

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Newbie | Next Rank: 10 Posts
Posts: 8
Joined: Fri Mar 13, 2015 11:07 pm
Followed by:1 members

Race timing

by Vincelauret » Fri Mar 20, 2015 2:44 am
In a 100m race , Ross completed 50m in 5 seconds. If he had increased his actual speed over the remaining 50m by 20 percent, what would have been his time for the race?
A. 10.66 seconds
B. 10.55 seconds
C. 10.44 seconds
D. 9.63 seconds
E. 8.8 seconds

I found that the answer is 9,16 seconds but it is not in the list above

If 50m=5S so the average speed is 10m/S

So for the remaining 50m if the speed is increased by 20% the calculation is:
10*1,2=12m/S => 50/12=4,16

so the time for the race is 5s+4,16s=9,16s

the correction guide of my book indicate that the good answer is [D]

Could someone explain me that difference?
Thanks[/spoiler]
Top
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 2663
Joined: Wed Jan 14, 2015 8:25 am
Location: Boston, MA
Thanked: 1153 times
Followed by:128 members
GMAT Score:770

by DavidG@VeritasPrep » Fri Mar 20, 2015 3:58 am
It would seem that your guide is mistaken - I think you did this correctly. What is the source?
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course
Top
User avatar
Newbie | Next Rank: 10 Posts
Posts: 8
Joined: Fri Mar 13, 2015 11:07 pm
Followed by:1 members

by Vincelauret » Fri Mar 20, 2015 4:02 am
A sample of question from the Business Aptitude Test, page 21 of the link below,
https://www.isb.edu/sites/default/files/ ... ooklet.pdf
Top
User avatar
Legendary Member
Posts: 2663
Joined: Wed Jan 14, 2015 8:25 am
Location: Boston, MA
Thanked: 1153 times
Followed by:128 members
GMAT Score:770

by DavidG@VeritasPrep » Fri Mar 20, 2015 4:22 am
I see. The question is referring to the bar graph presented. We see there that he finished race 1 in 10.55 seconds. If he ran the first 50 meters in 5 seconds, he ran the second 50 meters in 5.55 seconds. So his rate for the second half was 50/5.55, or about 100/11. (My estimation here means the correct answer will be slightly greater than what I calculate.)

So that 20% increase doesn't mean that we want the scenario in which he ran the second half of the race 20% faster than he ran the first half of the race. It means that we want the scenario in which he ran the second half of the race 20% faster than his recorded speed for the last 50 meters, 100/11. A 20% increase would be 120/11.

Time for the last 50 meters:
120/11 * t = 50
t = 550/120 = 55/12 = 4 + 7/12 = 4.583
5 seconds for first 50 meters + 4.583 for second 50 meters = 9.583 total
(Again, when I estimated the rate, I came up with a number that was slightly greater than the actual rate, so my time is a little smaller than the actual time.)

Closest to D.
Last edited by DavidG@VeritasPrep on Fri Mar 20, 2015 5:32 am, edited 1 time in total.
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course
Top
User avatar
Legendary Member
Posts: 2663
Joined: Wed Jan 14, 2015 8:25 am
Location: Boston, MA
Thanked: 1153 times
Followed by:128 members
GMAT Score:770

by DavidG@VeritasPrep » Fri Mar 20, 2015 4:31 am
(For anyone else following along, the question refers to the bar graph below)


Image
Veritas Prep | GMAT Instructor

Veritas Prep Reviews
Save $100 off any live Veritas Prep GMAT Course
Top
GMAT Instructor
Posts: 2630
Joined: Wed Sep 12, 2012 3:32 pm
Location: East Bay all the way
Thanked: 625 times
Followed by:119 members
GMAT Score:780

by Matt@VeritasPrep » Sun Mar 22, 2015 10:56 pm
It's closer to 4.63 than that, although the answer is still approximate.

From the graph, Ross completed Race 1 in 10.55 seconds. Since he completed the first 50 meters in 5 seconds, he completed the second 50 meters in 5.55 seconds.

D = RT

50 = R * 5.55

5000 = 555R

5000/555 = 1000/111 ≈ 9 = R

So Ross's rate was very close to 9 meters per second. Increasing this by 20% gives us 10.8 meters per second. To find the time it would take to do the second half of the race at this rate, we have

D = RT
50 = 10.8T
500 = 108T
500/108 = 125/27 = 4 + 17/27 ≈ 4.63 seconds
Top
User avatar
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 » Mon Mar 23, 2015 4:02 am
In a 100m race, Ross completed the first 50m in 5 seconds. His total time to complete the race was 10.55 seconds. If he had increased his actual speed over the remaining 50m by 20 percent, what would have been his total time for the race?

A. 10.66 seconds
B. 10.55 seconds
C. 10.44 seconds
D. 9.63 seconds
E. 8.8 seconds
Actual time for remaining 50m = (total time) - (time for the first 50m) = 10.55 - 5 = 5.55 = 555/100 = 111/20 seconds.

Faster time:
Rate and time are RECIPROCALS.
If Ross increases his speed for the second 50m by 20% -- implying that he travels at 120% = 6/5 of his actual speed -- he will take 5/6 of his actual time for the second 50m:
(5/6)(111/20) = 111/24 = 37/8 = 4.625 seconds.
Thus:
Total faster time = (actual time for first 50m) + (faster time for second 50m) = 5 + 4.625 = 9.625 seconds.

The correct answer is D.

Note that phrasing the decimals as FRACTIONS makes the calculations easier.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Master | Next Rank: 500 Posts
Posts: 126
Joined: Sat Jun 07, 2014 5:26 am
Thanked: 3 times

by sandipgumtya » Mon Mar 23, 2015 8:17 pm
Hi Vincelauret,
Do u have more sample practice papers?Really would b thankful.

Sandip.
Top
Post new topic Post Reply
• Page 1 of 1