Frequency of nth harmonic is 2lnv=400⇒0.6nv=400...(1)
And frequency of (n+1)th harmonic is 2l(n+1)v=450⇒0.6(n+1)v=450...(2), here, v is speed of wave on string.
From equation (1), we have
n=v400×0.6
Putting in equation (2),
⇒[v0.6×400+1]0.6v=450
⇒[400+0.6v]=450
⇒v=30ms−1
Speed of wave on string is stated as v=μT=30, here, μ is linear mass density.
So, μ=v2T=9002700⇒μ=3kgm−1