Answer
$0.846\;kg$
Work Step by Step
Tension in the string: $T=mg$
Frequency: $f=120\;Hz$
Wavelength: $\lambda=\frac{L}{2}=\frac{1.20}{2}\;m=0.60\;m$
Linear density of the string: $\mu=1.6\;g/m=1.6\times10^{-3}\;kg/m$
Therefore, the speed of the waves on the string:
$v=\sqrt {\frac{T}{\mu}}$
or, $f\lambda=\sqrt {\frac{mg}{\mu}}$
or, $f^2\lambda^2=\frac{mg}{\mu}$
or, $m=\frac{f^2\lambda^2\mu}{g}$
Substituting the given values
$m=\frac{120^2\times0.60^2\times1.6\times10^{-3}}{9.81}\;kg$
or, $\boxed{m=0.846\;kg}$
