How many miles could you drive for $3.90 if the gas mileage of your car is 14 km/liter and the price of gas is $0.32/gal? (This problem would model reality in 1974 but not quite now ...)

The first step is to find how many gallons of gas you can purchase:

3.90 divided by 32 = about 12

So, you can buy gas 12 times.

Next, how many litres is in that?

One gallon is 3.785 litres so the formula would be to multiply the value of gallons by 3.785

in this case, it's:

12 x 3.785 = 45.42

Now that we know how many litres you can buy, we simply have to mind how much we can drive with that amount of gas. If you can drive 14 km with one litre, we simply have to multiply the amount of litres of gas we have by 14.

(14 x 45.42 = 635.88)

Therefore, you can drive 634.88km with $3.90.