Three successive resonant frequencies of a string are 90, 150 and 210 Hz. If the length of the string is 80 cm, the speed of the transverse wave in the string is..........................

The least common multiple of resonant frequencies is 30 and it represent the fundamental frequency.
here odd multiples  $\Rightarrow$ string is free at one end.
$\therefore v=\frac{1}{4l}\sqrt{\frac{T}{\mu }}$ 
$\therefore 30=\frac{1}{4\times 0.8}\sqrt{\frac{T}{\mu }}$ 
$\Rightarrow \sqrt{\frac{T}{\mu }}=1.6\times 60=96m/s$