Show that there are exactly two reduced positive definite integer binary quadratic forms with discriminant $-20$; write these forms down.

State a criterion for an odd integer $n$ to be properly represented by a positive definite integer binary quadratic form of given discriminant $d$.

Describe, in terms of congruences modulo 20, which primes other than 2,5 are properly represented by the form $x^{2}+5 y^{2}$, and justify your answer.