The 20 pupils in a class each send 10 cards to 10 (different) class members. [size=92][i][note: you cannot send a card to yourself.][/i][/size]
(a) Show at least 2 pupils sent each other a card.
(b) Now suppose we had $n$ pupils sending $m$ cards each. For which $(m,n)$ is the above true? (That is, find minimal $m(n)$ or maximal $n(m)$)
1. **Necessity:**
   - We need to show that if \( m \leq \frac{n-1}{2} \), then it is possible for no two pupils to send each other a card.
   - Consider the arrangement where pupil \( A_j \) sends cards to \( A_{1+j}, A_{2+j}, \ldots, A_{m+j} \) (indices are taken modulo \( n \)).
   - In this arrangement, for any two pupils \( A_i \) and \( A_j \) with \( i < j \), the difference \( j - i \) and \( n - (j - i) \) cannot both be greater than or equal to \( m \).
   - This is because \( j - i \leq n - 1 \) and if \( m \leq \frac{n-1}{2} \), then \( j - i \) and \( n - (j - i) \) cannot both be greater than \( m \).
   - Therefore, no two pupils \( A_i \) and \( A_j \) send cards to each other.

2. **Sufficiency:**
   - We need to show that if \( m > \frac{n-1}{2} \), then there must be at least two pupils who send each other a card.
   - Suppose each pupil \( A \) sends cards to \( m \) different pupils and receives cards from \( m \) different pupils.
   - The total number of sends and receives for each pupil is \( 2m \).
   - Since \( m > \frac{n-1}{2} \), we have \( 2m > n-1 \).
   - This implies that the total number of sends and receives exceeds the number of other pupils (which is \( n-1 \)).
   - Therefore, by the pigeonhole principle, there must be at least one pair of pupils who send cards to each other.

\(\blacksquare\)

The final answer is \( \boxed{ m > \frac{n-1}{2} } \)