Zoe needs to drive 879 miles in 24 hours. She will need seven rest bre

Question

Zoe needs to drive 879 miles in 24 hours. She will need seven rest breaks of a half hour each, during which she will not be driving. Approximately what speed, in miles per hour, will she need to average during the periods when she is driving to finish the journey in time?

A. 40
B. 41
C. 42
D. 43
E. 44

A. 40
B. 41
C. 42
D. 43
E. 44

Tango3 · Answer

7 breaks of half hour each means distance has to be covered within 20.5 hrs

D = s x t

879 = s x (41/2)

s is approx 42

IMO C

100mitra · Answer

30 mins 7 break = 3.5 hrs (24-3.5) = 20.5 hrs actual driving for 879 kms 
Speed = distance / time = (879/20.5) = 42.87 average speed
It's definitely not 43, as the avg speed has not crossed it.
 
Correct Option C

Posted from my mobile device

Bunuel · Answer

This question is a part of  Are You Up For the Challenge: 700 Level Questions  collection.

tbhauer · Answer

drive time = 24 hrs - breaks = 24 - (.5*7)= 20.5 hrs driving
879 mi/20.5 hrs = 42.87 mph
Ans = D

AllisonBell · Answer

Let’s see if we can solve this without long division! I like to use the  Understand, Plan, Solve  process.

Understand  
Building on what other posters have shared, seven rest breaks of 30 minutes = 3.5 hours of rest time, so 20.5 hours of driving time. (Side note - what a road trip!)

Note that the way the question is worded, the priority is that Zoe “finish the journey in time.” With five tightly spaced answers and a question that calls for approximation, we’ll want to make sure we pick a speed that errs on the side of getting Zoe there a little too fast, rather than not fast enough.

Plan  
I don’t like the looks of this long division. I see numbers in the choices, and I think multiplication will be easier here, so I’ll try  Working Backwards,  breaking up the arithmetic to make this process quicker and more accurate.

Solve  
 Start with B:  41 mph * 20.5 hours = 41 * 20 + 41*.5 = 41 * 2 * 10 + 41/2 = 82*10 + 20.5 = 820 + 20.5 = 840.5. Too low.

Jump to D : 43 mph * 20.5 hours = 43 * 20 + 43 *.5 = 43*2*10 + 43/2 = 86*10 + 21.5 = 860 + 21.5 = 881.5. Too high.

Ordinarily, I’d just pick C. *But* since we’re estimating, and we need to get Zoe to her destination in time, picking C carries the risk that the answer choice will be under the miles needed.

Test C:  42 mph * 20.5 hours = 42 * 20 + 42*.5 = 42*2*10 + 42/2 = 84*10 + 21 = 840 + 21 = 861. Too low.

Since we need Zoe to definitely get where she’s going, let’s pick the closest choice that’s over the mark, Answer D.

For more practice with Working Backwards, check out  ManhattanPrep's Free GMAT Starter Kit .

Happy studying,

Ally Bell
ManhattanPrep GMAT Instructor

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