How does the execution time of fibnum$($n$)$ depend on the execution time$$for fibnum$($n$-1)$ and fibnum$($n$-2)?$ Use this relationship to obtain an approximate$$formula for the execution time of fibnum$($n$)$ as a function of n$.$Estimate how long it would take your computer to compute fibnum$(50).$Warning: You probably do not want to actually run fibnum$(50).$