Answer
$20\,\text{m/s}$
Work Step by Step
From the figure, we see that
$2.5\lambda = 50\,\text{cm}$, where $\lambda$ is the wavelength.
$\implies \lambda=\frac{50\,\text{cm}}{2.5}=20\,\text{cm}=0.20\,\text{m}$
Wave speed $v=\lambda f$ where $f$ is the frequency.
$\implies v=(0.20\,\text{m})(100\,Hz)=20\,\text{m/s}$
