Change in gas phase reaction equilibrium when not all product and reactant are increased
Given the reaction $$\ce{2NOCl &lt;=&gt; 2NO + Cl2}$$  we can formulate the equilibrium as $$\mathrm{K} = \dfrac{\ce{[NO]^2[Cl2]}}{\ce{[NOCl]^2}}$$

The problem then asks "What will be the effect on equilibrium concentration of NOCl when equal moles of NOCl and NO are introduced in the mixture at constant temperature?"

Now if we add equal amounts of $\ce{NOCl}$ and $\ce{NO}$ we don't have to do it at the same time to end up with the final equilibrium. 

So let's add the $\ce{NOCl}$ first. We now have two possibilities (assuming that it is very unlikely that K=1 exactly). 


If K>1 then more $\ce{NO}$ and $\ce{Cl2}$ form. 
If K&lt;1 then less $\ce{NO}$ and $\ce{Cl2}$ form. 


Now let's add the $\ce{NO}$. This has to push the reaction to the left. 

So the net result is it depends. You have too have enough data to plug into the equilibrium expression and other equations to work the net result out.