Two dogsled teams raced across a 300 mile course in Wyoming.

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

Two dogsled teams raced across a 300 mile course in Wyoming.

by BTGmoderatorLU » Wed Dec 26, 2018 2:12 pm

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Source: GMAT Prep

Two dogsled teams raced across a 300-mile course in Wyoming. Team A finished the course in 3 fewer hours than team B. If team A's average speed was 5 mph greater than team B's, what was team B's average mph?

A. 12
B. 15
C. 18
D. 20
E. 25

The OA is D

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Source: Beat The GMAT — Problem Solving | Wed Dec 26, 2018 2:12 pm

Manasa3190: Junior | Next Rank: 30 Posts; Posts: 22; Joined: Sat Oct 07, 2017 7:45 am

by Manasa3190 » Sat Dec 29, 2018 8:01 pm

Given,
Total distance = 300 miles
Let team B speed be X
then, Team A speed = X+5
Time B - Time A = 3 hours
(300/x) - (300/(x+5)) = 3
x^2 +5*x - 500 =0
x = 20, -25
Speed can only be positive, so Speed of B is 20

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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 » Sun Dec 30, 2018 9:54 am

BTGmoderatorLU wrote:Source: GMAT Prep

Two dogsled teams raced across a 300-mile course in Wyoming. Team A finished the course in 3 fewer hours than team B. If team A's average speed was 5 mph greater than team B's, what was team B's average mph?

A. 12
B. 15
C. 18
D. 20
E. 25

We can PLUG IN THE ANSWERS, which represent B's speed.
When the correct answer is plugged in, B's time - A's time = 3 hours.
For each team:
Time = distance/speed = 300/speed.

B: B's speed = 15 mph, implying that A's speed = 15+5 = 20 mph
In this case, B's time - A's time = 300/15 - 300/20 = 20 - 15 = 5 hours.
Eliminate B.

D: B's speed = 20 mph, implying that A's speed = 20+5 = 25 mph
In this case, B's time - A's time = 300/20 - 300/25 = 15 - 12 = 3 hours.
Success!

The correct answer is D.

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8086; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

by Scott@TargetTestPrep » Thu Feb 07, 2019 6:28 pm

BTGmoderatorLU wrote:Source: GMAT Prep

Two dogsled teams raced across a 300-mile course in Wyoming. Team A finished the course in 3 fewer hours than team B. If team A's average speed was 5 mph greater than team B's, what was team B's average mph?

A. 12
B. 15
C. 18
D. 20
E. 25

The OA is D

Let's let the rate of team B be r. The time of Team A is 300/(r + 5) and the time of team B is 300/r; thus:

300/(r + 5) + 3 = 300/r

Multiplying by r(r+5), we have:

300r + 3r^2 + 15r = 300r + 1500

3r^2 + 15r - 1500 = 0

r^2 + 5r - 500 = 0

(r + 25)(r - 20) = 0

r = -25 or r = 20

Since r can't be negative, team B's average speed is 20 mph.

Answer: D

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