Explanation:
1/81 gallon for 1/4 mile.
what do I need to multiply 1/81 with to get 1 gallon ?
it is simple, right ? i need to multiply it by 81 to get 1 gallon.
in order to keep the relationship (ratio) unchanged, I need to multiply the other part (1/4 mile) with the same factor :
1/4 × 81 = 81/4 = 80/4 + 1/4 = 20 1/4 miles
so, at this rate, that car can travel 20 1/4 miles using 1 gallon of gas.