Answer
0.2 m/s.
Work Step by Step
There is an antinode at each side of the cup, and a node right in the middle. Looking at figure 11-41b, we see that successive antinodes are a half-wavelength apart, i.e., the wavelength is twice the cup diameter. The wavelength is 0.16 m.
The wave speed is given by $v=\lambda f$. The frequency is $\approx 1 Hz$.
Solve for $v=\lambda f \approx 0.2 m/s$.
