$**$Amazon is testing out its new drone delivery system. Throughout the day, the number of packages aa drone can hold is given by the equation $p(t)=\frac{16}{t^2}$ where $p$ is in hundreds of package and $t$ is in hours. Assume $t>0.$

a$)$ How fast is the Amazon drone delivering packages at $t=4$ seconds?

b$)$ At what value of $t$ will the drone be delivering packages at a rate of $0\mathrm{.}25$ packages per hour?