Please answer with details.

Suppose you expect to pay $20,000 for tuition per year at the end of the next four years. If the yield curve is flat at 2.1% and you can invest only in zero coupon bonds with maturity 1 and 5 years, what should be the dollar amount invested in the 1-year bond to immunize the tuition expenses? Round to the nearest 100 dollars. 39,600