There are quite a few questions I came across, where using option 1, the answer would come up with x = 2 or x = -2 hence insufficient. Using option 2, x = 2 or x = 5 hence insufficient. But the answer is explained as - if we add option 1 and 2 we see that the common value of x between the two is x = 2, hence option C is the right answer. Is this right approach ? I do not understand if the equations were simplified to come to 2 possible values of x, how can we simply ignore those just because we are considering both equations true.

Example -
What is the greatest common divisor of positive integers j and k ?

1) The greatest common divisor of 3j and 2k is 2
2) The greatest common divisor of 5j and k is 10