3-5/8-3/8=3-(5/8+3/8)=3-1=21又8/11-(8/11+2/7)=1又8/11-8/11-2/7=1-2/7=5/7
2+3/8+5/8-5/8-3/8=21+8/11-8/11-2/7=1-2/7=5/7