It looks like everyone in this thread has answered the question correctly, so I won't rehash any of those steps here. This DS question is based on a variation of the "overlapping sets" concept that you'll likely see on the GMAT once or twice. There are several different ways to solve this problem, but since this is a simple variation, I'll show you the simple math.
The idea is this: If a person is a member of BOTH groups, then that person has been "counted" twice (once for the first group and once for the second). When calculating the total number of people, you're NOT supposed to count people twice, so you have to mathematically remove that "second count."
Here's a simple example:
3 people total
1 person in group A
1 person in group B
1 person in BOTH group A and B
Total in group A = 2
Total in group B = 2
But that DOES NOT mean that there are 4 people; there are only 3.
In this type of question, the "math" formula is Total = Group A + Group B - BOTH = 2 + 2 - 1 = 3 people total
The same logic applies in this question.
Total = 17
Company K = 12
Company R = 8
Both = ?
Total = CompK + CompR - BOTH
17 = 12 + 8 - BOTH
17 = 20 - BOTH
3 = BOTH
GMAT assassins aren't born, they're made,
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