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

Redeem

If the diameter of each of the car's wheels is 20 inches, wh

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Legendary Member
Posts: 698
Joined: Tue Jul 21, 2015 12:12 am
Location: Noida, India
Thanked: 32 times
Followed by:26 members
GMAT Score:740

If the diameter of each of the car's wheels is 20 inches, wh

by richachampion » Wed Oct 05, 2016 7:26 pm
If the diameter of each of the car's wheels is 20 inches, what was the approximate average speed of the car in km/h if after the two-hour journey each wheel made 75,000 revolutions? (1 inch = 0.0254 meter)

A. 32
B. 41
C. 59
D. 74
E. 88

SPOILER C
R I C H A,
My GMAT Journey: 470 → 720 → 740
Target Score: 760+
[email protected]
1. Press thanks if you like my solution.
2. Contact me if you are not improving. (No Free Lunch!)
Top
Source: — Problem Solving |
User avatar
Senior | Next Rank: 100 Posts
Posts: 51
Joined: Fri Sep 09, 2016 11:59 pm
Thanked: 20 times

by MBA Challengers » Thu Oct 06, 2016 1:15 am
Hi richachampion,

This question involves some slightly tedious arithmetic calculations but since the options are well spaced out, suggest you use some approximations:

Conceptually, we are looking at the circumference of the wheel as a synonym for the revolution.

So, start with calculating how many inches are covered with 1 revolution of the wheel:

Pi * diameter = 3.14 * 20 (For convenience and paucity of time, round off pi to 3). This results in 60 inches travelled per revolution

Each inch = 0.0254 metres. Round it off to 0.025 metres per inch. Convert the inch/revolution to metre/revolution by multiplying 60 * 0.025 to result in 1.5. So, each wheel travels ~1.5 metres per revolution.

In 2 hours, the car's wheels have turned 75,000 revolutions. This implies that the car has covered 75000 * 1.5 metres in 2 hours. Since, we need to calculate km/hr only calculate 75 * 1.5.
75 * 1.5 = 112.5. So, the car has travelled 112.5 km in 2 hours.

However, speed is calculated on a km/hr basis. When you divide this by 2, you get 56.25. Since you were anyway working on approximations, the closest answer which you will pick would be 59 km/hr which is option C.
Log on to www.mbachallengers.com for
Easy strategic GMAT prep
For any queries mail us at [email protected]
Follow MBA Challengers on Facebook
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 » Thu Oct 06, 2016 2:51 am
richachampion wrote:If the diameter of each of the car's wheels is 20 inches, what was the approximate average speed of the car in km/h if after the two-hour journey each wheel made 75,000 revolutions? (1 inch = 0.0254 meter)

A. 32
B. 41
C. 59
D. 74
E. 88
1 revolution = 1 circumference = πd = 20π inches ≈ 60 inches.
75,000 revolutions = 75000*60 inches.
(75000 revolutions)/(2 hours) = (75000*60 inches)/(2 hours) * (254/10000 meters)/(1 inch) * (1 kilometer)/(1000 meters) = (75*6*254)/(2*1000) ≈ (75*3*250)/1000= (75*3)/4 = 225/4 = somewhat more than 50.

The correct answer is C.
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
Post new topic Post Reply
• Page 1 of 1