Answer
$RH_{22}=33\%$
Work Step by Step
From the given dew point, we know the partial pressure is the same as the saturated vapor pressure at $5^oC$
$PP_5=SVP_5=872Pa$
$RH_{22}=\frac{PP_{22}}{SVP_{22}}\times100\%$
$SVP_{22}$ is not given, so will can calculate it
$SVP_{22}=SVP_{20}+2\frac{SVP_{25}-SVP_{20}}{5}=2666Pa$
$RH_{22}=\frac{872Pa}{2666Pa}\times100\%=33\%$
Even though the volume does change, it has no affect on the relative humidity because the percent of air component to volume will be constant.