2.15-（5/6-1/12)
=2.15-（10/12-1/12)
=2.15-9/12
=2.15-3/4
=2.15-0.75
=1.4
5/17-4/51+2/3
=5*3/51-4/51+2*14/51
=(15-4+28)/51
=39/51
=13/17