85-84又7/8÷7 =85-84÷7-7/8÷7=85-12-1/8=72又7/85/17÷1又1/2+1/3÷3又2/5=5/17×2/3+1/3×5/17=5/17×(2/3+1/3)=5/17×1=5/17