Explanation

(b)
Let the required number is $x$.
According to given condition
$x = \sqrt{x} + 12$
$\Rightarrow x - 12 = \sqrt{x}$
$\Rightarrow x^{2} - 25x + 144 = 0$
$\Rightarrow x^{2} - 16x - 9x + 144 = 0$
$\Rightarrow x = 16,\ 9$
Since $x = 9$ does not hold the condition.
$\therefore x = 16$