Final answer:
Mary paid a simple interest rate of 5.33% on a $3,000 loan after repaying $3,120 nine months later.
Step-by-step explanation:
The question involves calculating the simple interest rate charged on a loan. Mary Toy borrowed $3,000 from her parents and repaid them a total of $3,120 after 9 months.
The interest she paid can be found by subtracting the original loan amount from the total repaid amount, which is $3,120 - $3,000 = $120. To find the yearly interest rate, we use the simple interest formula, which is Interest = Principal × Rate × Time. We rearrange this formula to solve for the rate (R) as follows: R = Interest / (Principal × Time).
Using the given information, we calculate R as $120 / ($3,000 × (9/12)) = $120 / $2,250 = 0.0533. To express this as a percentage, we multiply by 100, resulting in an interest rate of 5.33% per year. Remember, the time must be in years, so 9 months is converted to 9/12 years.
Therefore, the simple interest rate Mary paid her parents was 5.33%, rounded to two decimal places.