(3/20*(7/12-1/2)): (13/24: (7/12+1/2))

(3/20*(7/12-1/2)):(13/24:(7/12+1/2))=1/40
1. 7/12-1/2=7/12-6/12=1/12
2. 3/20*1/12=1/80
3. 7/12+1/2=7/12+6/12=13/12
4. 13/24:13/12=13/24*12/13=12/24=1/2
5. 1/80:1/2=1/80*2/1=1/40