Hello! First I would just look at the numbers without the zeros. So 28/7. This equals 4. Next cross out the same number of zeros on each side of the equation. In this instance, the 7 has one zero and the 28 has three so you can cross out one zero on each side. This leaves you with two zeros left over to add to the end of the 4. So you end up with 400.
I hope this helps!