John traveled the entire 50 km trip. If he travelled the fir

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Max@Math Revolution: Elite Legendary Member; Posts: 3991; Joined: Fri Jul 24, 2015 2:28 amFollowed by: 37 members; Location: Las Vegas, USA; Thanked: 19 times

by Max@Math Revolution » Fri Mar 02, 2018 12:13 am

[GMAT math practice question]

John traveled the entire 50 km trip. If he travelled the first 20 km of the trip at a constant rate 40 km per hour and the remaining trip at a constant rate 20 km per hour, what is his average speed, in km per hour?

A. 20 km/h
B. 23 km/h
C. 25 km/h
D. 26 km/h
E. 27 km/h

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16206; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Fri Mar 02, 2018 6:30 am

Max@Math Revolution wrote:[GMAT math practice question]

John traveled the entire 50 km trip. If he travelled the first 20 km of the trip at a constant rate 40 km per hour and the remaining trip at a constant rate 20 km per hour, what is his average speed, in km per hour?

A. 20 km/h
B. 23 km/h
C. 25 km/h
D. 26 km/h
E. 27 km/h

Average speed = total distance/total travel time

total distance = 50 km

total travel time = time spent driving at 40 kmh + time spent driving at 20 kmh
time = distance/speed
So, time spent driving at 40 kmh = 20/40 = 1/2 hour
And time spent driving at 20 kmh = 30/20 = 3/2 hours
So, TOTAL travel time = 1/2 + 3/2 = 4/2 = 2 hours

Average speed = 50/2 = 25 kmh

Answer: C

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Brent

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mbawisdom: Senior | Next Rank: 100 Posts; Posts: 93; Joined: Tue Dec 16, 2014 9:50 amFollowed by: 4 members; Location: London, UK; Thanked: 2 times; GMAT Score:770

by mbawisdom » Fri Mar 02, 2018 7:06 am

Max@Math Revolution wrote:[GMAT math practice question]

John traveled the entire 50 km trip. If he travelled the first 20 km of the trip at a constant rate 40 km per hour and the remaining trip at a constant rate 20 km per hour, what is his average speed, in km per hour?

A. 20 km/h
B. 23 km/h
C. 25 km/h
D. 26 km/h
E. 27 km/h

Leg 1:
T = D/S
T = 20/40 = 1/2

Leg 2:
T = D/S
T = 30/20 = 3/2

Total time = 1/2 + 3/2 = 2

Avg Speed = D/T = 50/2 = 25

Max@Math Revolution: Elite Legendary Member; Posts: 3991; Joined: Fri Jul 24, 2015 2:28 amFollowed by: 37 members; Location: Las Vegas, USA; Thanked: 19 times

by Max@Math Revolution » Sun Mar 04, 2018 6:18 pm

=>
Ave Speed = ( Total Distance ) / ( Total Time )
The total distance = 50 km.
Total time is 20 / 40 + 30 / 20 = 0.5 + 1.5 = 2
The average speed is 50 / 2 = 25.
Therefore, the answer is C.

Answer : C

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8246; Joined: Sat Apr 25, 2015 10:56 amFollowed by: 29 members; Location: Los Angeles, CA; Thanked: 43 times

by Scott@TargetTestPrep » Mon Jun 10, 2019 6:13 pm

Max@Math Revolution wrote:[GMAT math practice question]

John traveled the entire 50 km trip. If he travelled the first 20 km of the trip at a constant rate 40 km per hour and the remaining trip at a constant rate 20 km per hour, what is his average speed, in km per hour?

A. 20 km/h
B. 23 km/h
C. 25 km/h
D. 26 km/h
E. 27 km/h

We can use the equation: average = total distance/total time

50/(20/40+ 30/20)

50/(1/2 + 3/2)

50/(4/2) = 50/2 = 25

Answer: C

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