equilibrium

Firstly, this is a system of constant volume at constant pressure.
Refer to the expression: PV = nRT
Since V, R and T are constant， thus P (pressure) is directly proportion to n (number of moles).

However, in the question, it is necessary to calculate the partial pressures of the components, but not the number of moles. So, concentrate on the partial pressures.

When 1 mol of PCl5 is used, 1 mol of PCl3 and 1 mol of Cl2 are produced.
It was shown above that partial pressure is directly proportional to number of moles.
Hence, when the partial pressure of PCl5 is decreased by 1 atm, both of the partial pressures of PCl3 and Cl2 are increased by 1 atm.

Assume that the partial pressure of PCl5 is decreased by x atm at equilibrium, each of the partial pressures of PCl3 and Cl­2­ is increased by x atm.
The initial partial pressures of PCl5, PCl3 and Cl2 are 1 atm, 0 atm and 0 atm respectively.
Hence:
Equilibrium partial pressure of PCl5 = (1 - x) atm
Equilibrium partial pressure of PCl3 = (0 + x) atm = x atm
Equilibrium partial pressure of Cl2 = (0 + x) atm = x atm

The total pressure at equilibrium = [(1 - x) + x + x] atm = (1 + x) atm