1.计算: (1)24+44+56 (2)53+36+47
解: (1)24+44+56 =24+(44+56) =24+100=124 (2)53+36+47 =53+47+36 =(53+47)+36 =100+36=136 2.计算: (1)96+15 (2)52+69 解: (1)96+15 =96+(4+11) =(96+4)+11 =100+11 =111 (2)52+69 =(21+31)+69 =21+(31+69) =21+100=121 3.计算: (1)63+18+19 (2)28+28+28 解: (1)63+18+19 =60+2+1+18+19 =60+(2+18)+(1+19) =60+20+20=100 (2)28+28+28 =(28+2)+(28+2)+(28+2)-6 =30+30+30-6=90-6 =84 这样想:因为28+2=30可凑整,但最后要把多加的三个2减去. |
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