怎样用简便方法求出4,5,6.711,12,13.18,19,20.的和快

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怎样用简便方法求出4,5,6.711,12,13.18,19,20.的和快怎样用简便方法求出4,5,6.711,12,13.18,19,20.的和快怎样用简便方法求出4,5,6.711,12,13.1

怎样用简便方法求出4,5,6.711,12,13.18,19,20.的和快
怎样用简便方法求出4,5,6.711,12,13.18,19,20.的和

怎样用简便方法求出4,5,6.711,12,13.18,19,20.的和快
4+5+6=(4+6)+5=3*5=15
11+12+13=(11+13)+12=3*12=36
18+19+20=(18+20)+19=3*19=57