Answer
4 Hz.
Work Step by Step
Use the equation for the fundamental frequency of a tube closed at one end.
$$f=\frac{v}{4L}$$
In the hot tube, the speed of sound increases, so we have two different frequencies.
$$\frac{f_{31}}{f_{25}}=\frac{v_{31}}{v_{25}}$$
$$ f_{31} = f_{25}\frac{v_{31}}{v_{25}}$$
The beat frequency is the difference in frequencies.
$$\Delta f=f_{31}-f_{25}=f_{25}\left(\frac{v_{31}}{v_{25}}-1 \right)$$
$$\Delta f=(349Hz)\left(\frac{331+0.60(31.0)}{331+0.60(25.0)}-1 \right)=3.63Hz\approx4Hz$$
