I will use miles per gallon here. Use the metric system if you prefer.

Say you are controlling you car's fuel mileage and it has done 320 miles with 11.2 gallons of gasoline.

How many miles per gallon is your car's mileage?:

>>> 320/11.2
28.571428571428573

The last result, miles per galon (mpg), is stored in memory. To show it type the underscore _:

>>> _
28.571428571428573

If the car can do 28.571... miles with one gallon, how many gallons would it burn in a 632 miles trip?

>>> 632/_
22.119999999999997

Now what is stored in memory is the last result, the total gallons of fuel for the trip:

>>> _
22.119999999999997

How much would be spent in this trip if the price of a gallon of gasoline is US$ 2.27?

>>> _ * 2.27
50.212399999999995