10/21-0.5+1.25-1/7+1/3
=10/21-1/2+5/4-1/7+1/3
=20/42-21/42+5/4+1/7+1/3
=-2/84+105/84+1/7+1/3
=103/84-12/84+28/84
=119/84
=17/12
祝楼主学习进步,
望采纳