Hey Juanrlos922,
Adding probabilities indeed doesn't make much sense. That's why we should be multiplying probabilities, like Vorpal did.
His calculation shows the probability of NOT hitting your pair. For the first community card, there are 48 unknown cards remaining. Subtract your 6 pair outs from this, and you have 42 cards remaining that don't give you a pair. On every next community card, you subtract one from both.
So the chance of not hitting your pair is:
42/48 * 41/47 * 40/46 * 39/45 * 38/44 = ~49.7%
Which means the chance of hitting your pair is roughly 1 - 49.7% = 50.3%. Then of course there's a chance that the QQ will actually hit a set or or a straight or that AK will hit the straight or flush, which changes the equity a bit.
Also to continue for the example you gave, having a deck of 4 cards and 1 out, your chances of getting your 1 out are actually this, using the same logic as above:
1 - (3/4 * 2/3 * 1/2) = 75%
Hopefully this isn't too confusing! 
Getting a good understanding of how this works exactly certainly can help with becoming a better poker player. But if you are for example just starting out, it might not be overly important to learn all the ins and outs of the math behind it. But it certainly can't hurt either!
Just ask if you have any further questions 
Regards,
Tino