a)5/7+7/13+19/13+6/5+9/7+9/5=(5/7+9/7)+(17/13+19/13)+(6/5+9/5)=14/7+36/13+15/5
=2+36/13+3=(2+3)+36/13=5+36/13=36/65
b)1/11+2/11+3/11+4/11+5/11+6/11+7/11+8/11+9/11+10/11
=(1/11+9/11)+(2/11+8/11)+(3/11+7/11)+(4/11+6/11)+10/11+5/11
=10/11+10/11+10/11+10/11+10/11+5/11=55/11=5
c)1/21+2/21+3/21+4/21+5/21+...+17/21+18/21+19/21+20/21
=(1/21+19/21)+(2/21+18/21)+...+(8/21+12/21)+(9/21+1/21)
=20/21+20/21+20/21+20/21+20/21+20/21+20/21+20/21+20/21+20/21+20/21+10/21
=230/21