Vincen wrote:If Lucy walks to work at a rate of 4 miles per hour, but she walks home by the same route at a rate of 6 miles per hour, what is Lucy's average walking rate for the round trip?

A. 4.2 miles/hour
B. 4.4 miles/hour
C. 4.8 miles/hour
D. 5 miles/hour
E. 5.8 miles/hour

The OA is C.

How can I determine the solution here?

The average speed is not equal to the average of 4 and 6. We cannot take an arithmetic average.

Say the distance is 12 miles. I assumed the distance 12 miles since 12 is the LCM of 4 and 6.

Time taken while going to work = 12/4 = 3 hours;
Time taken while coming to home = 12/6 = 2 hours

Average speed = (Total distance) / (Total time) = (12 + 12) / (3 + 2) = 24/5 = 4.8 miles per hour

The correct answer: C

Hope this helps!

-Jay

Download free ebook: Manhattan Review GMAT Quantitative Question Bank Guide
_________________
Manhattan Review GMAT Prep

Locations: New York | Vienna | Kuala Lumpur | Sydney | and many more...

Schedule your free consultation with an experienced GMAT Prep Advisor! Click here.

Hi Vincen,

Since this question gives us just the two rates to work with, we can solve it by TESTing VALUES.

The two speeds are 4 miles/hour and 6 miles/hour, so we should choose a common multiple of those two numbers for the one-way distance. Let's TEST 12 miles.

Walking to work:
D = (R)(T)
12mi. = (4 miles/hr)(X hours)
3 hours = X

Walking from work:
D = (R)(T)
12mi. = (6 miles/hr)(Y hours)
2 hours = Y

Total Distance = 12 + 12 = 24 miles
Total Time = 3 + 2 = 5 hours
Average Speed (round-trip) = 24/5 = 4.8 miles/hour

Final Answer: C

GMAT assassins aren't born, they're made,
Rich

Vincen wrote: ↑

Sun Oct 08, 2017 11:28 am

If Lucy walks to work at a rate of 4 miles per hour, but she walks home by the same route at a rate of 6 miles per hour, what is Lucy's average walking rate for the round trip?

A. 4.2 miles/hour
B. 4.4 miles/hour
C. 4.8 miles/hour
D. 5 miles/hour
E. 5.8 miles/hour

The OA is C.

How can I determine the solution here?

We can use the equation:

average speed = (total distance)/(total time)

If we let distance each way = 12, we have:

Average = (12 + 12)/(12/4 + 12/6)

Average = 24/5 = 4.8 mph

Answer: C