I2.1 If $a, p, q$ are primes with $a<p$ and $a+p=q$, find the value of $a$.
' 2 ' is the only prime number which is even. All other primes are odd numbers.
If both $a$ and $p$ are odd, then $q$ must be even, which means that either $q$ is not a prime or $q=2$. Both cases lead to contradiction.
$$
\therefore a=2
$$