[tex]x + 3x + 5x + ... + 37x + 39x = 2000[/tex]
[tex]x(1 + 3 + 5 + ... + 37 + 39) = 2000[/tex]
[tex]1 + 3 + 5 + ... + 2n - 1 = {n}^{2} [/tex]
[tex]2n - 1 = 39[/tex]
[tex]2n = 39 + 1[/tex]
[tex]2n = 40 \: | \div 2[/tex]
[tex]n = 20[/tex]
[tex] {n}^{2} = {20}^{2} = 400[/tex]
[tex]x \times 400 = 2000 \: | \div 400[/tex]
[tex]x = 5[/tex]
