Answer
$v = 8.0~m/s$
Work Step by Step
$y(x,t) = 0.040~sin(5\pi x)~cos(40 \pi t)$
We can see that $\omega = 40 \pi$ and $k = 5\pi$
Note that $\omega$ and $k$ also have these values in the equations for the two traveling waves that interfere to produce this wave.
We can find the speed:
$v = \frac{\omega}{k}$
$v = \frac{40 \pi~s^{-1}}{5\pi~m^{-1}}$
$v = 8.0~m/s$
