4/5除8-4/15x1/8=1/10-1/30=1/15
3除6/7除7/9=3*7/6*9/7=9/2
54x（8/9-1/3-1/6）=48-18-9=21
6/15x4/7+1除16/5+3/7x6/15
=6/15*(4/7+3/7)+1*5/16
=6/15+5/16
=171/240