Prob. # $2]$ The piston$-$cylinder arrangement, shown below, contains air at an undetermined pressure. The piston has a mass of $20$ kg and a diameter of $20$ cm $.$ The spring exerts a downward force of $1\mathrm{.}5$ kN on the piston, and atmospheric pressure is $101\mathrm{.}3$ kPa $.$ Determine the pressure of the air. $($Ans$.155\mathrm{.}29$ kPaa $)$