1/3-(0.875*(2/13)+1除6又1/2除8)*1又6/7=1/3-(875/1000*(2/13)+1/(13/2)/8)*13/7
=1/3-(7/8*(2/13)+1/(13*4))*13/7=1/3-(7/(13*4)+1/(13*4))*13/7
=1/3-2/13*(13/7)=1/3-2/7=1/21