1/1*6 +1/6*11+ 1/11*16+ 1/16*21+ 1/21*26+1/26*31+...+1/51*56
=(1/5)*(1-1/6)+(1/5)*(1/6-/1/11)+…+(1/5)*(1/51-1/56)
=(1/5)(1-1/56)
=11/56