13/15-3/20-7/30=(13×4-3×3-7×2)/30=(52-9-14)/30=(43-14)/30=29/30
13/15-3/20-7/30=【13*4-3*3-7*2】/60=【52-9-14】/60=29/60
13/15-3/20-7/30=26/30-7/30-3/20=19/30-3/20=38/60-9/60=29/60
29/60
