The yield percentage corresponds to the actual quantity of products obtained over the theoretical amount of product that should be produced. It can be expressed with the following formula.
Now, the actual yield obtained is found experimentally, generally, it is a datum given to you.
The theoretical yield to be obtained will depend on the stoichiometry of the reaction. If we have for example the following reaction:
If they give us the moles of reactant A and ask us to find the moles of C that will be formed, we must see the ratio C to A, which in this case will be c/a and the moles of C formed will be:
By having the two values, the actual yield and the theoretical yield, we can find the percentage yield with the first equation.