1/3-（0.875乘2/13加1除以6.5除以8）乘1又6/7
=1/3-(7/8x2/13+1÷13/2÷8)x13/7
=1/3-(7/52+1/52)x13/7
=1/3-2/13x13/7
=1/3-2/7
=1/21
手机提问的朋友在客户端右上角评价点【满意】即可.