GMAT prep DS question

[mlane25269]

Hey guys! What did I do wrong on this question? I answered "C" but the OA is "E"

[Stuart@KaplanGMAT]

This question requires a lot more calculation than we usually see on the GMAT.

We know time, we want to know if distance is greater than 6 miles.

(1) r > 16 feet per second

Let's start by swapping to miles per hour.

16 f/s = 16*3600 f/h = 16*3600/5280 m/h = 120/11 = approx 11mph

(we could also approximate 3600/5280 as roughly 3600/5400 = 2/3, and then 2/3(16) is between 11 and 12.)

So, minimum r is about 11. This means that:

minimum D = (min r)*t = 11(1/2) = 5.5

If the minimum distance is 5.5 miles, then D could be both lower and higher than 6: insufficient.

(1) r < 18 f/s

Same start:

18 f/s = 18*3600 f/h = 18*3600/5280 m/h = a bit more than 18(2/3) = a bit more than 12 mph.

So, maximum r is a bit more than 12. This means that:

maximum D = (max r)*t = a bit more than 12(1/2) = a bit more than 6.

Well, if maximum distance is more than 6, then actual distance could be both lower and higher than 6: insufficient.

Together: we know that 5.5 < D < a bit more than 6. So, it could still be both lower and higher than 6: insufficient.

[durgesh79]

yes it does need some calculations... but i feel its better if we convert everthing to feet... so its multiplication only ... and we dont have to approximate ....

so 6 mile = 6*5280 = 31680

statement 1 16*30*60 = 28800
statement 2 18*30*60 = 32400

after that same logic as above..... Answer E.

[mlane25269]

ok this seems like a tricky question. Would this be considered a 700+ level question?

[Stuart@KaplanGMAT]

ok this seems like a tricky question. Would this be considered a 700+ level question?

To be honest, I'd be very surprised to see this at any level.

The question isn't hard, it just requires a lot of calculations and has no subtlety; it's very un-GMAT-esque.

[Ian Stewart]

ok this seems like a tricky question. Would this be considered a 700+ level question?

To be honest, I'd be very surprised to see this at any level.

The question isn't hard, it just requires a lot of calculations and has no subtlety; it's very un-GMAT-esque.

Except that it is a real GMAT question- it's from GMATPrep. Questions requiring 'units conversion' - converting from miles per hour to feet per second, for example- are common on the GMAT. Typically these conversions can be done quickly if you cancel your fractions well, but can be very time consuming if you don't.

I think there is some subtlety to this question, at least in the method adopted. We are given information in miles per hour in the question, and we are given two statements about feet per second in the statements. We can either convert both statements to miles per hour, or we can convert one speed, from the question stem, to feet per second. Converting the speed in the question to mph cuts our work in half. Assume his distance was exactly 6 miles: So, Carlos rode at 12 mph

= 12*5280/(60*60) feet/sec
= 528/5*6 f/s
= 176/10 f/s
= 17.6 f/s

This is quick if you cancel, not so quick if you don't.

Now, the question is just asking, did he ride faster than 17.6 f/s? Neither statement tells us this.

[mlane25269]

I noticed that the question has Carlos' "average" speed in both of the statements so maybe that alone is enough to want to pick "E" since we only know his "average" speed and not his actual speed.

[Ancientmtk]

Average speed is enough to determine the distance.