The reactions: PCl5(g)↔PCl3(g)+Cl2(g)  &  COCl2(g)↔CO(g)+Cl2(g) are simultaneously in equilibrium at constant volume in same vessel. A few moles of  CO(g) are introduced into the vessel. After some time, the new equilibrium concentration of

Addition of CO(g) will increase rate of backward reaction of  reaction COCl2(g)↔CO(g)+Cl2(g) , with this concentration of Cl2(g)  decreases, hence rate of decomposition of PCl5 increases , resulting the decrease in concentration of PCl5 .