You buy a newly issued 1,000 20% ten-year bond, redeemable at 1,100 and having yearly coupons, for 1,400. You immediately take a constant amount D from each coupon and deposits it in another account earning 8% effective annual interest, so as to accumulate the full amount of the premium the moment after the final deposit. Find D.

D should be 20.71 but I can't figure it out