The primer checker

You will write three small functions that check a primer against the rules of the lesson.

The idea. A good primer has 18 to 25 bases, 40 to 60 per cent G and C, and a G or C at its 3 prime end. Its melting temperature is $64.9 + 41(G + C - 16.4)/N$.

Your tasks.

1. Finish gc_percent. It gives the share of G and C bases, as a per cent.
2. Finish melting_temp. It gives the Tm with the formula of the lesson.
3. Finish good_primer. It is True only when the length, the share of G and C and the 3 prime end all follow the rules.