Q.4 plz, all of them..full explanation ! : )

Apatient is given 50 mg dose of medicine the medicines effectiveness decreases every hour at a constant rate of 40% what is the exponential decay function that models this scenario how much medicine will be left in the patients system after 2 hours

Can someone me out here number 4 plz

Apackage of five erasers cost 0. $.39 at this rate how many how much would 60 erasers cost in dollars and cents