The "plus t" portion of the equation is fairly straightforward, and is found in every equation apart from C, immediately ruling it out. Lets assume s=6. Then, one third of 6 would be 2. 2s = 12, and 12 is not equal to 2, so B is not the answer. 1/2 + 6 = 6 1/2, and 6 1/2 is not equal to 2, so D is not the answer. 6/3 = 2, which is one third of 6, so A is the answer.
To represent 1/3 of a number, we can simply divide this number by 3, meaning if s is the number, 1/3 of s would be s/3. Now to add t all we have to do is + t, making our answer s/3 + t
