4/5*1/4 +0.25除以5/21-25%的简算 4/5×1/4+0.25÷5/21-25%=4/5×1/4+1/4÷5/21-1/4=1/4×（4/5+21/5-1）=1/4×（5-1）=1/4×4=1
4/5x1/4 +0.25÷5/21-25%=1/4x(4/5+21/5-1)=1/4x4=1
