In order to divide fractions, reciprocals are useful. The reciprocal of a number is it's multiplicative inverse. For fractions, this can be obtained by exchanging the numerator with the denominator. The -1 key on the calculator does this as well. Whole numbers are nonnegative fractions with a denominator of 1.
Division is equivalent to multiplying by the reciprocal.
Enlarging a picture. Peter and Paul drove equally fast, but Mary drove twice as far. The Tigers are better hitters than the Cubs. Toshie is for the metric system because she will be taller in centimeters than in inches. If a proportion has a missing term, we can simply cross-multiply and solve for the missing term.