Air at a temperature of $20$C and $750$ mm Hg pressure has a relative humidity of $80\%.$ What is its percentage humidity? Vapour pressure of water at $20$C is $17\mathrm{.}5$ mm Hg$.$

$80\mathrm{.}38$

$80$

$79\mathrm{.}62$

$78\mathrm{.}51$