(81+83+85+87+89+91+93)/(82+84+86+88+90+92+94)=(87-6+87-4+87-2+87+87+2+87+4+87+6)/(88-6+88-4+88-2+88+88+2+88+4+88+6)=(7×87)/(7×88)=87/88
