The car is 9ft long; the model is 6in long. Find the ratio of the length of the car to the length of the model:
A ratio is unitless, so all measurements must be expressed in the same units.
The length of the car is 9ft=(9*12)inches=108inches.
Thus the ratio of the car to the model is 108:6. Simplifying yields 108:6=18:1 So the answer is (a) 18:1