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1、13和7,第一題乘積個位是1,1*1=1,3*7=21,9*9=81,所以這兩個數的末位一定是這幾組中的一個,又已知和為20,則一個是十位數,一個是個位數,所以鎖定答案13和7 2、一看就出來了是3和12
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