For this equation, we will be representing "a number" with x. You start with the part of the equation that includes either multiplication and/or division. In this case, it's "one third of a number." To take one-third of something is to divide it by 3, therefore we know that the first part of the equation must be x/3. The remaining bit of the equation is "increased by 12." The phrase "increase by" indicates addition, therefore we know the second half of the equation is +12. Put the two together to get your answer:
(x/3) + 12