2/3*7+2/7*11+2/11*15+...+2/39*43
=2*1/4(1/3*7+1/7*11+1/11*15+.1/39*43)
=1/2(1/3-1/7+1/7-1/11+1/11-1/15+.1/39-1/43)
=1/2*(1/3-1/43)
=20/129