Hi amandakay,

The GMAT would never expect you to calculate the exact value of 4024/5280, so if you're ever thinking about doing something so "calculation heavy", then there's almost certainly a faster or easier way to get to the correct answer. In this prompt, you should notice the word "approximately" - that means that you DON'T have to calculate that fraction, but you do have to estimate the value.

If we 'round down', then 4024 ft./5280 ft. is a little less than 4/5 of a mile. Knowing THAT, and that 20 mph is 1 mile every 3 minutes, can you correctly answer the question?

GMAT assassins aren't born, they're made,
Rich
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Video solution from Quant Reasoning starts at 13:37
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Here's another method that allowed me to solve the question in approximately 30 seconds:

20 miles per hour translates to traveling 1 mile every 3 minutes. Since the bridge is longer than 1/2 mile but shorter than 1 mile, the answer must be between 1.5 and 3 minutes. The only answer choice that fits this range is 2 minutes.

In this question unit of measurements have to be taken care of.

Distance: 4024 feet. This is (4024/5280) miles = 0.76 miles

Speed: 20 miles per hour

Time: (Distance / Speed) (0.76/20) {This will come in Hours}. Multiply by 60 to get answer in minutes. It is 2.28 minutes.

Answer to be 2 minutes.

First, convert the speed to feet per minute.

20 miles per hour = ( 20 * 5280 ) feet per 60 minutes.

Time = \(\frac{Distance}{Speed}\) = \(\frac{4024}{(20 * 5280 ) / 60}\) = \(\frac{4024 *60}{20 * 5280}\) = \(\frac{4024 * 3}{5280}\) = \(\frac{4024}{5280}\)*3

\(\frac{4024}{5280}\) < 1. Therefore, Time < 3.

We can eliminate (A) because \(\frac{4024}{5280}\) is not equal to \(\frac{1}{3}\). The only answer choice remaining is (B). Therefore, (B) is the answer.

distance = 4024 ft = 4024/5280 miles

time taken = 4024/5280*20 hours= 0.76 hours = 2.28 minutes close to 2 minutes

option B

Approximation!

D=4000
R=20mph
Convert it to miles per minutes = \(\frac{20}{60}\)
R = \(\frac{1}{3}\)

1 mile = approx 5000 feet

\(T=\frac{D}{R}\) \(=\frac{(4000*3)}{5000}\) \(=\frac{12000}{5000}\) \(=\frac{12}{5}\) = 2.4

Since the denominator is slightly more than 5000, the value will be less than 2.4.
i.e B

hello friends

here is a quick way to solve it

if 20 miles = 60 min---> then 1 mile = 3 min

if length of bridge (4,024 feet) is shorter than 1 mile (5,280 feet)

then only option is B

The length of the bridge, in miles, is:

4,024/5,280 = 503/660, which is about 500/660 = 50/66 = 25/33.

Using the formula: time = distance/rate, we see that the time, in hours, is:

[25/33]/20 = 25/(33 x 20) hours

So the time in minutes is:

25/(33 x 20) x 60 = (25 x 3)/33 = 25/11, which is about 2 minutes.

Answer: B
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20 miles per hour = 3 miles/min

4024 / 5280 = 4000 / 5000 = 4/5

4/5 * 3 = 12/5 = 2 minutes

Answer is B.
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Hi basshead,

Based on your various posts in the Quant forums, you are clearly a strong overall Quant thinker, but you have to be careful about the individual steps that you work through on each question that you face (since GMAT writers can often anticipate the type of little mistake that someone might make - and then the writers put the result of that mistake among the answer choices).

Here, 20 miles/hour is equivalent to 1 mile every 3 minutes (NOT 3 miles/minute).

GMAT assassins aren't born, they're made,
Rich
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If they would have picked 3 min as an option, it would have been a little harder.

1/3 of a mile in 1 min
1/3 of 5280 feet is around 1750 feet
then 2 min = 3500 feet

And 4 min would be way too much.