The missing piece here is the gallon consumption per hour

30 miles -> 1 gallon
50 miles -> 5/3 gallons

So, in 5 hours the car would have consumed 25/3 gallons.

Fraction with respect to the full tank is (25/3) / 12 = 25/36

sorry didnt understand how you came to

25/36 from (25/3)/12 =25/36

the way I look at it is, in order for us to how much gas he used we need to know the distance.

5 hours * 50 mph = 250 miles

also,

the amount that can be used by a 12 gallon tank is

30*12=360 miles

so the fraction used to the total is 250/360 - Thats my logic but it doesnt give me that. Can someone please help me understand this better.

The problem asks "the amount of gas used would be what fraction of a full tank??"

You have to find the fraction here:
Numerator = to be calculated
Denominator = 12 ( full tank of gas )


As jayhawlk mentions: one has to find out how much gallons is used for 5 hours.

From the problem
1 gallon = 30 miles @ 50 miles per hour
So to travel 50 miles in one hour, car consumes 50/30 = 5/3 gallons.

We need to find out what is the consumption for 5 hours = 5 * consumtioon for 1 hours = 5 * (50/30) = 25/3

So the final solution is (25/3)/12 = 25/36

thanks again for making it clear for me. I understand now.