（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
