1）原式=7÷8×1÷6÷7×9
=(7×9)/(7×6×8)
=3/16
2）原式=12×5/6+12×5/12
=10+5
=15
3）原式=20-1/5+1/5
=20
4）原式=18/13×1/39-5/13×1/39
=(18/13-5/13)×1/39
=1×1/39
=1/39