In this case we have an unkown variable that is a number. We will call this number by "x", this is what we want to calculate. In the first part it is said that the number or x is added to the numerator and in the second part it is said that twice the number is added to the denominator, and that this fraction is equal to 1/3. So we have:
When number is added to the numerator of 12/43, the number is 12+X/43 and twice the number is added to the denominator of 12/43, this will give 12+X/43+2X, the resuliting fraction is equivalent to 1/3. Mathematically, this will be
