Wow! That rate is awful!
30% per year is represented by the decimal fraction 0.30.
Roughly speaking, the interest rate per 30-day month is 30%/12, or 0.30/12, or 0.025.
Thus, the interest payable on this credit card is roughly 0.025 times the amount due.
If you owed $1000, you'd have to pay (0.025)($1000) in interest for that 30 day period. That'd be $25.