Let \( a \) be an integer such that \( \displaystyle \lim_{x \to 7} \frac{18 - [ 1 - x ]}{[x - 3a]} \) exits, where \( [ t ] \) is greatest integer \( \le t \). Then \( a \) is equal to:
If \( [t] \) denotes the greatest integer \( \le t \), then the number of points, at which the function \( f(x)=4|2x+3|+9\left[x+\dfrac{1}{2}\right]-12[x+20] \) is not differentiable in the open interval \( (-20,20) \), is ________.
