Marsha borrowed $300 for 18 months at 13 percent interest. How much will she pay in interest?

If Marsha had borrowed $300 with an interest of 13% for 18 months, then let's say that interest is for a year which will leave 18 months as 1.5 years. Multiply $300 to 0.13 (representing 13%) that will leave you with $39 (the interest amount) and multiply it by 1.5 years. Add it all up, the answer is $358.5