In an equilibrium, why are partial pressures not affected by the coefficients of the balanced equation?
One way of thinking about this is writing the original equation
$$\ce{2NOBr(g) &lt;=&gt; 2NO(g) + Br2(g)}$$
as
$$\ce{NOBr(g) + NOBr(g)  &lt;=&gt; NO(g) +NO(g) +  Br2(g)}$$
Now it should be clear you have to multiply, not add concentrations.