The rope shown at an instant is carrying a wave travelling towards right, created by a source vibrating at a frequency $n$. Consider the following statements

$I.$ 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

A string of $7 \;m$ length has a mass of $0.035\,kg$. If tension in the string is $60.5\; N,$ then speed of a wave on the string is …. $m/s$

If ${n_1},{n_2},{n_3}………$ are the frequencies of segments of a stretched string, the frequency $n$ of the string is given by

If $n _{1}, n_{2}$ and $n _{3}$ are the fundamental frequencies of three segments into which a string is divided, then the original fundamental frequency $n$ of the string is given by

Two open organ pipes of fundamental frequencies $n_{1}$ and $n_{2}$ are joined in series. The fundamental frequecny of the new pipe so obtained will be