Find all n ∈ N so that 16 ≡ 25 (mod n).

To solve 16 ≡ 25 (mod n), we need to find all natural numbers n such that the remainder of 16 divided by n is equal to the remainder of 25 divided by n.

Let's use the definition of congruence modulo n:
a ≡ b (mod n) if and only if n divides (b - a).

In this case, we have:
16 ≡ 25 (mod n) if and only if n divides (25 - 16) = 9.

Now we need to find all natural numbers n that divide 9, which are the divisors of 9.
The divisors of 9 are 1, 3, and 9.

Therefore, the solutions for n are n = 1, n = 3, or n = 9.

$\boxed{n = 1, 3, 9}$ Answer.