The frequency of the nth harmonic is related to the fundamental harmonic by the equation:
[tex]f_n=nf_1[/tex]In this case we know the fifth harmonic, that is n=5 and f5=450, then we have:
[tex]\begin{gathered} 450=5f_1 \\ f_1=\frac{450}{5} \\ f_1=90 \end{gathered}[/tex]Therefore, the fundamental harmonic is 90 Hz