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At a constant speed of 72 kilometers per hour, a vehicle

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by [email protected] » Sat Oct 20, 2018 11:12 am
Hi All,

We're told that a vehicles travels at a constant speed of 72 kilometers per HOUR. We're asked what that speed is in kilometers per SECOND. This question is really just about 'unit conversions', although at a certain point you can use the 'spread' of the answer choices to your advantage and stop doing 'math.'

1 hour = 60 minutes
1 minute = 60 seconds
1 hour = (60)(60) = 3600 seconds

72 km/3600 seconds

Since 72/720 = .1, we know that 72/3600 is far LOWER than .1
There's only one answer that matches...

Final Answer: A

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by ceilidh.erickson » Tue Oct 23, 2018 8:20 am
Here's an easy way to set up UNIT CONVERSIONS - set it up as fractions, so that numerators and denominators of the same unit cancel:

Image

Image

Now there's no confusion about whether to multiply or divide, etc. Now simplify: take a 6 out of each 60, and two 6's out of 72:

Image

We're left with 2 km every 100 sec, or 0.02 km/sec.

The answer is A.
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by ceilidh.erickson » Tue Oct 23, 2018 8:25 am
For more on doing unit conversions correctly, see:
https://www.manhattanprep.com/gmat/blog ... ike-a-pro/
https://www.manhattanprep.com/gmat/blog ... our-units/
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At a constant speed of 72 kilometers per hour, a vehicle

by fskilnik@GMATH » Tue Oct 23, 2018 10:33 am
Hi, Ceilidh!

I am glad to know the Manhattan also considers UNITS CONTROL as one of its prep tools.

(You call it UNITS CONVERSIONS but we are talking about the very same thing!)

As you have already realized in this forum, this is one of the most powerful (and used) tools we offer in the GMATH method.

(We have extended the basic function it usually has to a WIDE diversity of GMAT situations!)

Kind Regards,
Fabio.
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by ceilidh.erickson » Tue Oct 23, 2018 12:10 pm
"Units Control" has a great space-age ring to it, like something you'd shout from the bridge on Star Trek! I like it.
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by fskilnik@GMATH » Tue Oct 23, 2018 12:23 pm
ceilidh.erickson wrote:"Units Control" has a great space-age ring to it, like something you'd shout from the bridge on Star Trek! I like it.
Love that, Ceilidh! :)

I created the expression because "values follow units", in other words, the units themselves "take control"...

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by Scott@TargetTestPrep » Thu Oct 25, 2018 8:23 am
swerve wrote:At a constant speed of 72 kilometers per hour, a vehicle travels how many kilometers per second?

A. 0.02
B. 0.2
C. 0.6
D. 1.2
E. 6.0
Since 1 hour = 3600 seconds, 72 km/hour = 72/3600 = 1/50 = 0.02 km/second.

Answer: A

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