一.5/11×3/4+6/11÷4/3+0.75
=(5/11+6/11)*3/4+3/4
=3/4+3/4
=3/2
二.1/4÷（1-1/2-1/4）
= 1/4÷1/4
=1
三.7/8×5/13+7/13×3/8
=7/8*(5/13+3/13)
=7/8*8/13
=7/13
四.（9又1/4-3.2）÷2又1/2+1.68
=(6+1/4-1/5)÷5/2+1.68
=121/20*2/5+1.68
=1.42+1.68
=3.1