$15)$ A mass is oscillating on a spring with a period of $4\mathrm{.}60$ s$.$ At t $=0$ s the mass has zero speed and is at x $=8\mathrm{.}30$ cm$.$ What is the value of t the first time after t $=0$ s that the mass is at x $=4\mathrm{.}15$ cm$?$