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 $n(A \cap B) = 5$. 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 partitioned into two disjoint subsets: $A \cap B$ and $A \cap B'$. Therefore, $$ n(A) = n(A \cap B) + n(A \cap B') $$ 3. **Substitute known values:** $$ 20 = 5 + n(A \cap B') $$ 4. **Solve for $n(A \cap B')$:** $$ n(A \cap B') = 20 - 5 = 15 $$ 5. **Interpretation:** There are 15 elements in the subset of $A$ that are not in $B$. **Final answer:** $$ n(A \cap B') = 15 $$