Let $ a $ denote the number of chocolate candies, $ b $ the number of vanilla, $ c $ the number of peppermint, and $ d $ the number of lemon. We can represent the information given in the problem with the following system of linear equations: \begin{align*} a+b+c+d &= 15 \\ 2(a+b) &= c+d \\ c-8 &= d \end{align*} Substituting for $ c+d $ in terms of $ a+b $ into the first equation gives $ 3a + 3b = 15 $, or $ a + b = 5 $. This means that $ c + d = 10 $. The third equation can also be expressed as $ c - d = 8 $. Adding these two equations gives $ 2c = 18 $, so $ c = 9 $. Because $ d = c - 8 $, $ d = \boxed{1} $.