1/5+1/10+1/20+-----+1/160
=1/5+1/10+1/20+-----+1/160+1/160-1/160(1/160+1/160=1/80)
=1/5+1/10+1/20+-----+1/80+1/80-1/160(1/80+1/80=1/40)
=2/5-1/160
=63/160
1/5+1/10+1/20+-----+1/160
=1/5+1/10+1/20+-----+1/160+1/160-1/160(1/160+1/160=1/80)
=1/5+1/10+1/20+-----+1/80+1/80-1/160(1/80+1/80=1/40)
=2/5-1/160
=63/160