=2/3*7+2/7*11+……+2/39*43
=(1/2)*(1/3-1/7)+(1/2)*(1/7-1/11)+……+(1/2)*(1/39-1/43)
=(1/2)*(1/3-1/7+1/7-1/11+……+1/39-1/43)
=(1/2)*(1/3-1/43)
=20/129