Answer :

To simplify the ration 300 to 1000, we need to divide both by the largest common factor these two numbers share. In this case, this would be a 100. After dividing by a 100, we'd get the following ratio: 3 to 10
Cars to People =  300 : 1000

=  300 / 1000  =  3/10.    (The two zeros at the top would cancel the two at the bottom)

That's the fraction in simplest form.