28的所有因数
60以内8的所有倍数
①1-= ② ③ ④ ⑤ ⑥0.2+=
642÷6= 0.9×99+0.9= 1.25×80= 2.5÷0.01=
÷ = 2﹣ ﹣ = 3﹣ ﹣ = ( ÷ )×30=