To write a ratio as a fraction, we simply take the first number in the ratio and make it the numerator while taking the second number in the ratio and making it the denominator.
Because we want the ratio of engines to box cars, our ratio should be:
number of engines/number of box cars
When we substitute in our respective values, we get:
4/18
To simplify this ratio, we have to find the GCF, or greatest common factor of the numerator and the denominator, which in this case is 2. To simplify, we divide both the numerator and the denominator by the GCF, as follows:
4/2 / 18/2
When we simplify, we get:
2/9
Therefore, your answer is 2/9.
Hope this helps!