The fundamental frequency of vibration of a string between two...

The fundamental frequency is $f = 50Hz$.

The length of the string is $l = \dfrac{\lambda}{2}$

Linear mass density is $\mu = \dfrac{m}{l} = 20g{m}^{- 1}$

Using $v = f\lambda$

$\dfrac{v}{2l} = 50$

$⇒ v = 100 \times l = \dfrac{100 \times 18}{20} = 90m{s}^{- 1}$