Apply:
[tex]Q=Q_0e^{-\frac{t}{RC}}[/tex]where:
RC= time constant = 40.4 s
Q0 = initial charge
Q = final charge
[tex]\frac{Q_0}{2}=Q_0e^{-t/RC}[/tex][tex]\frac{Q_0}{2Q_0}=e^{-t/RC}[/tex][tex]\frac{1}{2}=e^{-t/RC}[/tex][tex]ln(\frac{1}{2})=-\frac{t}{RC}[/tex][tex]ln(\frac{1}{2})=-\frac{t}{40.4}[/tex][tex]-0.693\text{ = -}\frac{t}{40.4}[/tex][tex]t=\text{ 28 s}[/tex]