1. We know that Pierre is 3 years older than Claude, so we can write:
p = c + 3
where p is Pierre's age and c is Claude's age.
2. To find out how old Pierre is when Claude is 17, we can substitute 17 for c in the equation we just wrote:
p = c + 3
p = 17 + 3
p = 20
Therefore, Pierre is 20 years old when Claude is 17 years old.