To reach Sarah's goal of having $20,000 in 4 years, we'll use the formula for the present value of a future sum (PV):
PV = FV / (1 + i)^n
Here, FV is the future value ($20,000), i is the interest rate per period (0.045/12), and n is the number of periods (4 years * 12 months/year).
PV = $20,000 / (1 + 0.045/12)^(4*12)
PV ≈ $16,711
So, Sarah needs to deposit approximately $16,711 into the 4-year CD to reach her goal of having $20,000 in 4 years. The answer is:
$16,711