Final answer:
The percent of interest Johnny pays on his $100 loan for two weeks is 23%, calculated using the simple interest formula and converting the loan term to a fraction of a year.
Step-by-step explanation:
To calculate the percent of interest paid on the loan, we can use the simple interest formula:
Interest = Principal × Rate × Time
In Johnny's case, he took out a $100 loan and agreed to pay back $109 in two weeks. The interest he pays is the difference between what he repays and the original loan amount, which is $109 - $100 = $9.
To find the interest rate, we divide the interest by the principal amount and the time period. However, since the loan is for two weeks, we need to express the time period as a fraction of a year. There are 52 weeks in a year, so two weeks is 2/52 of a year.
The formula to find the interest rate is:
Rate = Interest / (Principal × Time)
And substituting the values we have:
Rate = $9 / ($100 × 2/52)
This gives us a rate of 0.234 or 23.4% when expressed as a percentage. Rounded to the nearest whole percent, the interest rate is 23%.