$($Points: $20)$ How many years is a flexible pavement designed to last given the following conditions:

Daily traffic: $9,567$ cars $($each with two $2-$kip single axles$),57$ buses $($each with two $20-$kip single axles$),$ and $157$ heayy trucks $($each with one $12-$kip single axle and two $34-$kip tandem axles$)$

$7-$inch sand$-$mix asphaltic surface

$8-$inch crushed stone base with a drainage coeficient of $0\mathrm{.}9$

$8-$inch crushed stone subbase with a drainage coefificient of $0\mathrm{.}95$

Subgrade CBR of $5\mathrm{.}5$

Overall standard deviation of $0\mathrm{.}45$

Reliability of $90\%$

Initial PSI of $4\mathrm{.}2$ and final PSI of $2\mathrm{.}5$