Final answer:
The extra cost paid by taking the buy now pay later plan is equivalent to a yearly interest rate of 25%.
Step-by-step explanation:
To calculate the extra cost paid by taking the buy now pay later plan, we first need to calculate the total payments made under the plan. Sarah paid $100 down payment and $25 per month for 12 months. So the total payments made would be $100 + ($25 x 12) = $400.
Now, we subtract the original cost of the lawnmower ($320) from the total payments made under the plan ($400) to calculate the extra cost paid. $400 - $320 = $80.
Next, we need to calculate the actual yearly rate of interest equivalent to this extra cost. To do this, we divide the extra cost ($80) by the original cost of the lawnmower ($320) and multiply by 100 to convert it into a percentage. ($80 / $320) x 100 = 25%.
Therefore, the actual yearly rate of interest equivalent to the extra cost paid is 25% (option C).