用同一句口诀“七八五十六”．

三七二十一

{#blank#}1{#/blank#}×3，{#blank#}2{#/blank#}×7．

5×5=                             3×2=                                  6×6=

2×2=                             4×5=                                  1×4=

7×3=                            4×7=                                  4×4=

28÷4=                           35÷5=                                42÷7=

56÷8=                           36÷9=                                21÷3=

三三{#blank#}1{#/blank#}        二五{#blank#}2{#/blank#}         二八{#blank#}3{#/blank#}

三{#blank#}4{#/blank#}十五      {#blank#}5{#/blank#}九四十五     {#blank#}6{#/blank#}二十八

{#blank#}1{#/blank#}×4<29       45>5×{#blank#}2{#/blank#}         7×{#blank#}3{#/blank#}<50