Credit card with30%interest rate for 30 day period

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.