1. **State the problem:** We are given a universal set $U$ with $n(U) = 100$, and two subsets $A$ and $B$ with $n(A) = 20$, $n(B) = 60$, and their intersection $n(A \cap B) = 10$. We need to find the number of elements in the disjoint subset $n(A \cap B')$. 2. **Recall the formula:** The set $A$ can be divided into two disjoint parts: the intersection with $B$ and the part of $A$ outside $B$. This is expressed as: $$n(A) = n(A \cap B) + n(A \cap B')$$ 3. **Substitute the known values:** $$20 = 10 + n(A \cap B')$$ 4. **Solve for $n(A \cap B')$:** $$n(A \cap B') = 20 - 10$$ $$n(A \cap B') = 10$$ 5. **Interpretation:** There are 10 elements in the subset of $A$ that are not in $B$. **Final answer:** $$n(A \cap B') = 10$$