An Old GMAT 700+ Problem

[wcheng57]

Another PS question from an old GMAT Test Paper exam. I can't for the life of me get B as the answer.


Question:

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

A) 10.5%

B) 12.5%

C) 15%

D) 22%

E) 30%


The correct answer is B, but I have no explanation. Can anyone help?

[liferocks]

Let total 100x motorists exceeded the speed limit
so motorists who exceed the posted speed limit do not receive speeding tickets=20x
And he motorists exceed the posted speed limit and receive speeding tickets=100x-20x=80x
now this is i.e 80x=10% of total motorists
so total number of motorists=800x
hence % of the motorists onthe road exceed the posted speed limit=(100x/800x)*100%=12.5%

Ans option B

[frank1]

Lol.... i sometime feel if they allow me 75x2 minutes i may be able to solve all question....(just time wise and thinking wise....)
i am saying this in this question becoz...
once i got time to think over this question,it turned out to be very simple question

ok wording of the question makes it confusing.....
solution is if we suppose total number of vechile 100 then
x be who exceeded
80% of x=10 find value of x that is 12.5

explanation
(where can thing go wrong)
that 10% is of total 100
but that 20% is of that (10% of 100)....not of total 100

so 20% of people who exceeds were lucky so 80% were fined
those
80% of x=10
x=12.5
done...
once we understand things
things end in 80% of x=10