Coi chieu dai hinh chu nhat cu la 100% thi chieu dai hinh chu nhat moi so voi chieu dai hinh chu nhat moi chiem :
100% -20% =80%
Chieu rong hinh chu nhat moi so voi chieu rong hinh chu nhat cu chiem:
100%:80%=1.25
1.25 =125%
De dien h khong thay doi thi phai tang chieu rong len so phan tram la:
125%-100%=25%
Dap so 25%