（68-17/18）×1/17
=68×1/17-17/18×1/17
=4-1/18
=3又17/18
（1-3/4）×8/15+5/6
=1×8/15-3/4×8/15-5/6
=8/15-2/5-5/6
=16/30-12/30-25/30
=-21/30
=-7/10