The question is "18 is 45% of what number?".

                                     (Write the proportion)

                                   (Substitute: 18 for a and 45 for p)

                              (Cross products property)

                                    (Multiply: )

                               (Divide each side by 45)

                                      (Cancel common terms)

                                             (Divide: )

18 is 45% of 40.