x+3x+5x+...+1997x = 2x+4x+6x+....+1996x+9990
x·(1+3+5+...+1997) = x·(2+4+6+...+1996) + 9990
x·(1+3+5+...+1997) - x·(2+4+6+...+1996) = 9990
x·(1-0+3-2+5-4+7-6+...+1997-1996) = 9990
x·(1+1+1+...+1) = 9990 {1 de (1997+1)/2 = 999 ori}
x·999 = 9990
x = 9990:999