GMATPrep Percentage problem

[jaydeer44]

Does anyone know how to solve the following GMATPrep problem?

On a certain road 10 percent of the motorists exceed the posted speed limit and receive speeding tickets, but 20 percent of the motorists who exceed the posted speed limit do not receive speeding tickets. What percent of the motorists on the road exceed the posted speed limit?

(A) 10.5%
(B) 12.5%
(C) 15%
(D) 22%
(E) 30%

The answer is B.

[Bharat]

Meaning: many people exceeed speed limit & from these overspeeding folks a few get tickets. We have to locate total how many people exceed the limit.

1. 20% of overspeeding people do not get the ticket, i.e., only 80% of overspeeding people get the ticket.
2. total 10% motorists get ticket.

now combine these two:

80% (ticketed people) = 10% of all motorists on the road.
100% of ticked people = (10*100/80)% of all motorists

this means that total of 12.5% of all motorists exceed speed limit.

Let me know in case of doubts/mistakes, if any.

Regards,
Bharat.

[jaydeer44]

Thanks a lot Bharat!

[adity_13]

I had the same confusion. It is clear now. Thanks

[Brent@GMATPrepNow]

On a certain road 10 percent of the motorists exceed the posted speed limit and receive speeding tickets, but 20 percent of the motorists who exceed the posted speed limit do not receive speeding tickets. What percent of the motorists on the road exceed the posted speed limit?

(A) 10.5%
(B) 12.5%
(C) 15%
(D) 22%
(E) 30%

The answer is B.

We can also use the Double Matrix Method to solve this question.

This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of motorists, and the two characteristics are:
- speeder (S) or non-speeder (~S)
- get ticket (T) or not get ticket (~T)

Aside: To learn more about the Double Matrix Method, watch our free video: https://www.gmatprepnow.com/module/gmat- ... ems?id=919

Since this question concerns percents (instead of actual values), let's assign a "nice" value to the total number of motorists in this population. Let's say there are 100 motorists.

So, to begin, our matrix looks like this.


10 percent of the motorists exceed the posted speed limit and receive speeding tickets
The top left box is for motorists who speed and receive speeding tickets. So, 10% of the entire population will be in this box.


20 percent of the motorists who exceed the posted speed limit do not receive speeding tickets.
The motorists referred to here are those who go in the top right box. Unfortunately, we don't know the total number of speeders, so we can't find 20% of that value.
So, let's let x = the total number of speeders.


Now we can deal with this: 20 percent of the motorists who exceed the posted speed limit do not receive speeding tickets.
In other words, 20% of x will go in the top right box.


At this point, we know that the sum of the top 2 boxes is x.
So, we can write: 10 + 0.2x = x (now solve)
Arrange: 10 = 0.8x
Divide: 10/0.8 = x
12.5 = x

Since x represents the total number of speeders, we know that 12.5 out of 100 motorists speed.
In other words, [spoiler]12.5%[/spoiler] of motorists speed.
Answer: B

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Once you're familiar with the Double Matrix Method, you can attempt these additional practice questions:

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Difficult Data Sufficiency questions
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Cheers,
Brent