22. Five mark example problem V

A mobile phone is put to use when the battery power is $100 \%$.The percent of battery power '$y$' remaining after using the mobile phone for $x$ hours is assumed as $y = -0.25x + 1$.

 

(i) Draw the graph of the equation.

 

(ii) Find the number of hours elapsed if the battery power is $40 \%$.  hours.

 

(iii) How much time does it take so that the battery has no power?  hours.