If `U={p,q,r}` and `A={p}` then `n(A')` =________

Answer

To solve the problem, we need to find the number of elements in the complement of set A, denoted as A'.

### Step-by-Step Solution:

1. **Identify the Universal Set (U)**:
The universal set is given as \( U = \{p, q, r\} \). This set contains all the elements we are considering.

2. **Identify Set A**:
The set A is given as \( A = \{p\} \). This set contains only one element, which is 'p'.

3. **Determine the Complement of Set A (A')**:
The complement of set A, denoted as A', consists of all the elements in the universal set U that are not in set A.
- Since A contains 'p', we will remove 'p' from U.
- Therefore, \( A' = U - A = \{p, q, r\} - \{p\} = \{q, r\} \).

4. **Count the Number of Elements in A'**:
Now, we need to count the elements in A'.
- The elements in A' are 'q' and 'r'.
- Thus, the number of elements in A' is \( n(A') = 2 \).

### Final Answer:
The number of elements in A' is \( n(A') = 2 \).

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