An air$-$water vapour mixture has a dry bulb temperature of $60$C and a dew point temperature of $40$C$.$ The total pressure is $101\mathrm{.}3$ kPa and the vapour pressure of water at $40$C and $60$C are $7\mathrm{.}30$ kPa and $19\mathrm{.}91$ kPa respectively. The humidity of air sample expressed as kg of water vapour$/$kg of dry air is:

$0\mathrm{.}048$

$0\mathrm{.}079$

$0\mathrm{.}122$

$0\mathrm{.}152$