Thomas charged $2,400 on his credit card to buy furniture. the interest rate charged by the credit card company is 24.9%. how much will he pay in interest after one month?

I=prt
Interest = principal X rate X time
You are asking for interest so lets fill in all the variables.

i=2400X.249X(1/12)
I used .249 for rate because you convert the interest rate to a decimal.
I used 1/12 for time because you only need 1 month and time is based off of a year.  For example, if you wanted to know how much interest after 3 months you would use (3/12)
Now solve.
i=2400X.249X.0833333
i=49.80
Your interest after one month is $49.80