At temp T K,PCl5 is 50% disociated at an equili pressure of 4 atm.At what pressure it would dissociate to the extent of 80% at the same temp.?

The reaction can be written as:
PCl5(g)...................=.....PCl3(g)...+......Cl2(g)
p-p/2.............................p/2..................p/2.........at T
equilibrium pressure=p-p/2+p/2+p/2=4 atm
Now calculate p and then Kp.
let at P’ pressure, the dissociation is 80% while while temperature remains constant, so Kp remaains constant.
p-0.8p.............................0.8p........0.8p
Now write the total pressure=p’-0.8p’+0.8p’+0.8p’=P’
Now use expression for Kp and calculate p’ and then P’.