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Need help on this

by sohaib_iba » Tue Jun 01, 2010 9:55 am
In city X, 80% of the households have cars. Of these 20% run their cars on fuel and the rest on CNG. What percent of the people in city X run their cars on CNG?

A) 64%
B) 80%
C) 36%
D) 20%
E) 40%
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by indiantiger » Tue Jun 01, 2010 10:11 am
Total population = n

Total people using car (N') = 80% of n

total people using car with cng as fuel = 80% of N'

= 80*80 /100*100 *n= (64/100) *n

% of total people using cnd car =( 64n/100 )/n *100 = 64% (A)

Let me know if I did any step wrong.
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by selango » Tue Jun 01, 2010 10:14 am
Lets number of people in city X be 100

Number of people who have cars=80/100*100=80

Numbers of people with cars on fuel=20/100*80=16

Numbers of people with cars on CNG=80-16=64

Percentage of people with cars on CNG=64/100=64%
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by selango » Tue Jun 01, 2010 10:17 am
indiantiger wrote:Total population = n

Total people using car (N') = 80% of n

total people using car with cng as fuel = 80% of N'

= 80*80 /100*100 *n= (64/100) *n -->from this step itself we can identify the percentage.Its 64% of n.

% of total people using cnd car =( 64n/100 )/n *100 = 64% (A)

Let me know if I did any step wrong.
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by indiantiger » Tue Jun 01, 2010 11:01 am
@selango,

Agreed, I was trying to be more thorough, right all the steps.
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by Testluv » Tue Jun 01, 2010 6:11 pm
As selango points out, when dealing with percents of unspecified values, picking 100 is a fast way of attacking the problem (if the algebra doesn't jump off the screen):

....................................................100 households.............
....................................80 with cars..................20 without cars
..........................64with cng...and 16 on fuel ........................................................(20% of 80 is 16)

Because 64 is 64% of 100, the answer is 64%.
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