In my opinion, reduction of fractions to a common denominator (12*13*14*15*16*17*18*19*20 + 11*13*14*15*16*17*18*19*20 + 11*12*14*15*16*17*18*19*20 + ... 11*12*13*14*15*16*17*18*19) / 11*12*13*14*15*16*17*18*19*20 then, sum of the denominator is (12*13*14*15*16*17*18*19*20 + 12*13*14*15*16*17*18*19*...