1) 5/7×3/5=3/7
2) 5/6-3/8-1/12=60/72-27/72-6/72=27/72=3/8
3) 21/20-3/8-1/5=42/40-15/40-8/40=19/40
4) 13/15-(1/5+1/6)=13/15-(6/30-5/30)=13/15-1/30=26/30-1/30=25/30=5/6
