=(1/3)*(1/3-1/6)+(1/3)*(1/6-1/9)+(1/3)*(1/9-1/12)+(1/3)*(1/12-1/15)+(1/3)*(1/15-1/18)
=(1/3)*(1/3-1/6+1/6-1/9+1/9-1/12+1/12-1/15+1/15-1/18)
=(1/3)*(1/3-1/18)
=5/54
=(1/3)*(1/3-1/6)+(1/3)*(1/6-1/9)+(1/3)*(1/9-1/12)+(1/3)*(1/12-1/15)+(1/3)*(1/15-1/18)
=(1/3)*(1/3-1/6+1/6-1/9+1/9-1/12+1/12-1/15+1/15-1/18)
=(1/3)*(1/3-1/18)
=5/54