38÷19/40=（ 80 ）
7/20×99+7/20
=7/20 *(99+1)
=7/20 *100
=35
（1-3/4）×8/15+5/6
=1/4 *8/15 +5/6
=8/60 +50/60
=58/60
=29/30
5/7×[（3/4-1/2）×4/5]
=5/7 *[(3/4 -2/4)*4/5]
=5/7*[1/4 *4/5]
=5/7 *1/5
=1/7
[（31/39-18/41）×0]÷12/13
=0÷12/13
=0