the first thing to note is since one gallon= 60 miles then 1/2 gallon= 30 miles, now I know 1/2 is not an answer but it is easiest to decrease in moderate amounts. 1/2 gallon= 48 Km so we are close, and 1/2=0.5 so if we brought it down to 0.42 we would have 25.2 miles and 25.2 miles = 40.32. The question wants to know the approximate number and 40.32 is the closest we are going to get so the answer is 0.42
