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.

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%

@selango,

Agreed, I was trying to be more thorough, right all the steps.

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%.