Assuming that the frequency is it's fundamental frequency we have that:
n=1[tex]\mu=\frac{Tn^2}{4f^2L^2} , n=1,L=150,f=20\\ [/tex]
If the string is being pulled by a hanging mass then we have that:
[tex]T=Mg[/tex]
where M is the hanging object mass. Then we get that M is:[tex]\frac{\mu.4.f^2.L^2}{g}[/tex]
