Final answer:
To find the interest rate, use the simple interest formula, where the interest earned is the total amount minus the principal. Convert the time period to years, then solve the rearranged formula to get the interest rate.
Step-by-step explanation:
The quickest way to find the unknown interest rate (r) from the given equation would be to apply the simple interest formula. We are given the total amount (A), principal amount (P), and the time period in months (t). The simple interest formula is I = P × r × t, where I is the interest earned.
To solve for r, first calculate the actual interest earned by subtracting the principal (P) from the total amount (A), which yields I = A - P. Next, convert the time period from months to years if it's not already in years, because interest rates are typically annual rates. In this case, 4 months would be ⅓ of a year. Finally, rearrange the equation to solve for r: r = I / (P × t). Plug in your values and calculate r.