Let \( X = \{ x \in N : 1 \le x \le 19 \} \) and for some \( a, b \in R \), \( Y = \{ ax + b : x \in X \} \). If the mean and variance of the elements of \( Y \) are \( 30 \) and \( 750 \), respectively, then the sum of all possible values of \( b \) is:
