gives us $\frac{{2\pi }}{\lambda } = \frac{\pi }{{15}} \Rightarrow \lambda = 30$
Distance between nearest node and antinodes
= $\frac{\lambda }{4} = \frac{{30}}{4} = 7.5$
$I.$ The speed of the wave is $4n \times ab$
$II.$ The medium at $a$ will be in the same phase as $d$ after $\frac{4}{{3n}}s$
$III.$ The phase difference between $b$ and $e$ is $\frac{{3\pi }}{2}$
Which of these statements are correct