负60×(3/4+1/6-7/12)=-60x3/4-60x1/6+60x7/12=-45-10+35=-20
-60×(3/4+1/6-7/12)=-60×1/3=-20-60×(3/4+1/6-7/12)=-60×3/4-60×1/6+60×7/12=-45-10+35=-20
