700?25=()?100=()

700?25=?2800100=?28

a/b=(a*n)/(b*n)=(a/n)/(b/n)

700?25

=700?425?4?

=2800?100

=28

？

()(?)

68?17?17=68(?68?17?17=68)?

320?(2?5?8)=320?2?5?8=4?

56?(8?4)=56?8?4=28?