Chapter 4.3, Problem 100E

Charging a Battery The rate at which a battery change is slower the closer the battery is to its maximum change $c_{\circ }$ The time (in hours ) required to change a fully discharged battery to a charge C is given by
$t=-In(1-\frac{c}{c_{\circ }})$
Where k is a positive constant that depends on the battery.
For a certain battery, $k=0.25$ . If this battery is fully discharged, how long will it take to change to 90% of its maximum charge $c_{\circ }$ ?