1. **State the problem:** We are given the sizes of sets $A$, $B$, their union $A \cup B$, and the universal set $U$. We need to find the number of elements in the subset $A \cap B'$ (elements in $A$ but not in $B$). 2. **Recall the formula for union of two sets:** $$n(A \cup B) = n(A) + n(B) - n(A \cap B)$$ 3. **Calculate $n(A \cap B)$:** $$80 = 30 + 70 - n(A \cap B)$$ $$n(A \cap B) = 30 + 70 - 80 = 20$$ 4. **Find $n(A \cap B')$:** This is the part of $A$ that does not intersect with $B$. $$n(A \cap B') = n(A) - n(A \cap B) = 30 - 20 = 10$$ **Final answer:** $$\boxed{10}$$