if A=70 and B= 79, how much is A union B

There is not enough information... and A and B can't be this big... probabilities are within 0 \leq P \leq 1, except if you intended 70% and 79% respectively, which you didn't.

We must know how big is P(A \cap B)

If P(A \cap B) = 70%, then P(A \cup B) = 79%

If P(A \cap B) = 49%, then P(A \cup B) = 100%