25
先试探.1x3x5x7x9 = 45 (mod100) ,
1x3x5x7x...x99 = (1x3x5x7x9)^10+(0+1+2+...+9)x10x(1x3x5x7x9)^9x(1x3x5x7+1x3x5x9+1x3x7x9+1x5x7x9+3x5x7x9) = (1x3x5x7x9)^10 = 45^10 = 25 ^ 5 = 25 (mod100),
因此 1x3x5x...x999999 = (1x3x5x7x...x99)^10000 = 25 ^ 10000 = 25 (mod100)