39*(1/4)+(3/4)*(21)
=39/4+63/4
=(39+63)/4
=102/4
=51/2是简便方法吗39*(1/4)+(3/4)*(21)=39*(1/4)+(1/4)*(3*21)=39*(1/4)+63*(1/4)=(39+63)*(1/4)=102/4=51/2=25.5