Two datasets, A and B, are combined. Dataset A has $n_A = 10$ and a mean $\bar{x}_A = 20$. Dataset B has $n_B = 15$ and a mean $\bar{x}_B = 30$. If the combined dataset has a total sum of squares $\sum (x_i - \bar{x}_{combined})^2 = 450$ and the variance of dataset A is $s_A^2 = 4$ and the variance of dataset B is $s_B^2 = 6$, what is the variance of the combined dataset?