How to solve?

Hi,

what's the source of this question? It can't be from the actual GMAT (or GMAT materials), since there's only 4 answer choices. Also, the language is unidiomatic (at least for North American English).

In any case, we know that Mark's age is equidistant between John's and Jim's. If we wanted to solve algebraically, we'd say:

Mark - Jim = John - Mark

2(Mark) = John + Jim

Since we know that John + Jim = 48, we now know that:

2(Mark) = 48
Mark = 24

Of course, we didn't really need to use algebra if we used two of the most valuable tools in our GMAT arsenal: common sense and logic. If Mark is right in between John and Jim, then Mark's age must the average of John's and Jim's. Since:

Average = (sum of terms)/(# of terms)

we know that:

Mark = (48)/(2) = 24.