Answer
$v = 30~m/s$
Work Step by Step
We can see that this is the fourth harmonic since there are 4 antinodes. We can find the wavelength.
$\lambda = \frac{2L}{n}$
$\lambda = \frac{(2)(0.60~m)}{4}$
$\lambda = 0.30~m$
We can find the wave speed as:
$v = f~\lambda$
$v = (100~Hz)(0.30~m)$
$v = 30~m/s$