Answer
$0.83m$
Work Step by Step
The required distance can be determined as follows:
We know that the distance from a node to an anti node is equal to the quarter of a wavelength $\lambda$; that is: $L=\frac{\lambda}{4}$ and for the third harmonic mode $L=\frac{3\lambda}{4}$
Now $d=\frac{1}{3}L$
We plug in the known values to obtain:
$d=\frac{1}{3}(2.5m)$
$\implies d=0.83m$
