Final answer:
To work backward from the fraction 1/3, you should multiply it by 3 to get back the original number. If you have a part of something and know it is 1/3, multiplying it by 3 gives you the whole of that thing.
Step-by-step explanation:
To work backward from the fraction 1/3 to check the answer, you need to perform the inverse operation of the one used to get to 1/3. In basic mathematics, if you have divided a number to arrive at a fraction, you would multiply to go back to the original number. So, if you have 1/3 as a result of division, you would multiply by 3 to get back to the whole. This confirms that the correct answer is (a) Multiply by 3.
To illustrate this with an example, imagine you have a pie and you cut it into three equal parts. Each part is 1/3 of the pie. To find out the size of the whole pie, you would combine the three pieces, essentially multiplying the fraction 1/3 by 3, which equals 1, representing the whole pie.