Units$-$of$-$activity depreciation

A truck acquired at a cost of $$320,000$ has an estimated residual value of $$20,000,$ has an estimated useful life of $40,000$ miles, and was driven $3,200$ miles during the year. Determine the following. If required, round your answer for the depreciation rate to two decimal places.

$($a$)$ The depreciable cost

$($b$)$ The depreciation rate

$($c$)$ The units$-$of$-$activity depreciation for the year