Answer
$18^{\circ}C$.
Work Step by Step
From Table 13–3, at $25^{\circ}C$, the saturated vapor pressure is 23.8 torr. The relative humidity is 0.65, so the partial pressure of water is 0.65 of 23.8 torr, or 15.47 torr.
Find the dew point, the temperature where the saturated vapor pressure is 15.47 torr.
That is no entry in Table 13–3 for that, but we may linearly interpolate between $15^{\circ}C$ and $20^{\circ}C$ for the unknown temperature T. .
$$\frac{T-15^{\circ}C }{20^{\circ}C -15^{\circ}C }=\frac{15.47torr-12.8torr}{17.5torr-12.8torr}$$
$$T=17.84^{\circ}C\approx 18^{\circ}C$$