Suppose that each of the 5 persons knows a piece of information, each piece is different, about a certain event. Each time person $A$ calls person $B$, $A$ gives $B$ all the information that $A$ knows at that moment about the event, while $B$ does not say to $A$ anything that he knew.

(a) What is the minimum number of calls are necessary so that everyone knows about the event?
(b) How many calls are necessary if there were $n$ persons?
Let's break down the problem and solution step-by-step.

### Part (a): Minimum Number of Calls for 5 Persons

1. **Initial Setup**:
   - There are 5 persons, each knowing a unique piece of information.
   - We need to ensure that every person knows all 5 pieces of information.

2. **Sufficient Condition**:
   - Choose a fixed individual, say person \( A \).
   - Have everyone else call \( A \). This requires \( 4 \) calls (since there are 4 other persons).
   - After these 4 calls, \( A \) knows all 5 pieces of information.
   - Now, \( A \) calls each of the other 4 persons to share all the information. This requires another \( 4 \) calls.
   - Total calls: \( 4 + 4 = 8 \).

3. **Necessary Condition**:
   - Consider the last time a piece of information, say \( a \), is shared for the first time.
   - This must be no earlier than the \( 5 \)th call because there are 5 pieces of information, and each must be shared at least once.
   - After this call, \( a \) is known by 2 people.
   - To ensure \( a \) is known by all 5 people, we need at least \( 5 - 2 = 3 \) more calls.
   - Thus, the total minimum number of calls is \( 5 + 3 = 8 \).

### Part (b): Minimum Number of Calls for \( n \) Persons

1. **Sufficient Condition**:
   - Choose a fixed individual, say person \( A \).
   - Have everyone else call \( A \). This requires \( n-1 \) calls.
   - After these \( n-1 \) calls, \( A \) knows all \( n \) pieces of information.
   - Now, \( A \) calls each of the other \( n-1 \) persons to share all the information. This requires another \( n-1 \) calls.
   - Total calls: \( (n-1) + (n-1) = 2n - 2 \).

2. **Necessary Condition**:
   - Consider the last time a piece of information, say \( a \), is shared for the first time.
   - This must be no earlier than the \( n \)th call because there are \( n \) pieces of information, and each must be shared at least once.
   - After this call, \( a \) is known by 2 people.
   - To ensure \( a \) is known by all \( n \) people, we need at least \( n - 2 \) more calls.
   - Thus, the total minimum number of calls is \( n + (n - 2) = 2n - 2 \).

The final answer is \( \boxed{ 2n - 2 } \).