Notice that the information you are given is "dollars per kilometer per bus" or , and "children per buses" or and that you are asked for "dollars per child" or .
Since you want "dollars" in the numerator, and it is already in the numerator of [tex]\frac{dollars}{(kilometers)(buses)}, so you want to multiply by that fraction. Also, you want "children" in the denominator, and it is in the numerator of , you want to divide by that fraction (to divide by a fraction you invert and multiply.
Okay, just that would give you . The "buses" in the numerator and denominator cancel leaving . In order to get rid of the "kilometers" left in the denominator, we need to multiply by "kilometers".
That is why everyone is telling you to multiply "1.35 dollars per kilometer per bus" by "50 kilometers" and divide by "children per bus".
That is assuming that every bus is filled. other wise, we would need to know the precise number of children.