When multiplying to cancel out units, a unit is canceled when in one term it is on top and in another term it is on the bottom.
So in the first blank we are needing to cancel out miles (mi) so that it does not end up in the final answer. Since it is on the bottom in the second term, we need it to be on the top in the first blank - therefore the first blank is:
Then when we go to find out what all of what we multiplied equals, we must go back and cancel out units that are both on the top and the bottom, as they are not to be included in the final answer.
So as we can see, with
in the first blank, miles (mi) are cancelled, then in the 2nd and 3rd terms yards (yd) is on the top and on the bottom, so that is cancelled out, and then we are only left with feet (ft) on the top. Therefore, feet will be the only unit that is left in the equation and that is the final blank: ft.