A spring with spring constant $15 \mathrm{N} / \mathrm{m}$ hangs from the ceiling. A ball is attached to the spring and allowed to come to rest. It is then pulled down $6.0 \mathrm{cm}$ and released. If the ball makes 30 oscillations in $20 \mathrm{s}$, what are its (a) mass and (b) maximum speed?