15分之3-6分之1+6分之5
=1/5+4/6
=1/5+2/3
=13/15
25-12分之5-18分之7
=25-(15/36+14/36)
=25-29/36
=24又36分之7
3分之2+6分之1-12分之5
=8/12+2/12-5/12
=5/12