(a/b) -(c/d) = (ad -bc)/(bd) . . . . a useful formula to remember when working problems like this. It is equivalent to multiplying the first fraction by d/d and the second one by b/b to get a common denominator of bd. Of course if the second fraction is added instead of subtracted, the sign changes in both places.
31.57 cars is the answer rounded to the hundredth, but since its cars you might want to put just 31 since half a car cant pass through. it depends on what your teacher wants really.
