The extent of reaction is a useful quantity in computations with equilibrium reactions. Consider the reaction 
:2 A ⇌ B + 3 C
where the initial amounts are , and the equilibrium amount of A is 0.5 mol. We can calculate the extent of reaction in equilibrium from its definition
In the above, we note that the stoichiometric number of a reactant is negative. Now when we know the extent, we can rearrange the equation and calculate the equilibrium amounts of B and C.