18 5/12-7/12∙119/21-17/72∙2/3

7/12×119/21=119/36=3 11/3617/72×2/3=17/10818 5/12-3 11/36-17/108==18 45/108-3 33/108-17/108==15 12/108-17/108==14 120/108-17/108==14 103/108