Dividing by a fraction is equivalent to multiply by its inverse. For example, if you want to divide 3/4 by 5/6, you have to compute 3/4 multiplied by 6/5
18/20 can be simplified by dividing each term by the same number, in this case by 2.
If you want to divide mixed numbers, first, you have to convert the mixed numbers into improper fractions, and then proceed as explained above.