3.75-[(-8又3/7)+(-7.5)+(-21又4/7)+(3又1/2)]+0.25
=3.75+0.25-[-8-3/7-7-1/2-21-4/7+3+1/2]
=4-(3-8-7-21-3/7-4/7)
=4-(3-37)
=4+34
=38