Pivot games

What is the probability that in randomized quick-sort , a random pivot selection on an input of $n$ keys leads to recursive calls, both of which are no smaller than $\dfrac{n}{ 16}$ ?

If the probability can be expressed as $\dfrac{a}{b}$ , where $a$ and $b$ are coprime positive integers . Input $a+b$ as your answer.

Assume the keys are uniform throughout.