If we simply it, then it would be
3 + 2x + 6 + x = 15, which equals
3x + 6 + 3 = 15,
3x + 9 = 15,
if we subtract 9 from 15, we get 6. If we divide 3x by 3, and 6 by 3, we get 2.
Therefore x = 2, and we know there is a 10/15 chance to get a humbug, or a 1/3 chance.
Hope this helped.