You are missing the distance or time that the car traveled at 5mi/hr.

sorry, you right

At a speed of 50 miles per hour, a certain car uses 1 gallon of gasoline every 30 miles. If the car starts with a full 12 gallon tank of gasoline and travels for 5 hour at 50 miles per hour, the amount of gasoline used would be what fraction of a full tank ?

This problem is just manipulating units. We will be multiplying using ratios to cancel units to get what we want.

We are looking for a ratio of gallons used to initial gallons (12). So we need to find the number of gallons used.

It looks cleaner on paper. When I write (50/1)(mi/hr) here, I write it on my paper as:

50 mi
_____
1 hr

So we have (50/1)(mi/hr) * (1/30)(gal/mi) = (50/30)(gal/hr) = (5/3)(gal/hr).

We traveled for 5 hours so (5/3)(gal/hr) * 5(hr) = (25/3)gal. This is the numerator of our ratio. The denominator is the 12 gallons we started with. So our ratio is (25/3)/12 (gal/gal) = (25/3)*(1/12) = 25/36.

allegation:

50*30/1=50*150/x
x=25/3
so fraction is 25/(3*12)
25/36

many thks