Acute renal graft rejection can occur years after the graft. In one study (The Lancet, December 24, 1994, page 1737), 21 patients showed such late acute rejection when the ages of their grafts (in years) were 9, 2, 7, 1, 4, 7, 9, 6, 2, 3, 7, 6, 2, 3, 1, 2, 3, 1, 1, 2, and 7, respectively. The mean and the standard deviation are 4 and 2.8. Assume that ages of renal grafts are normally distributed. What is the margin of error for constructing a 90% confidence interval estimate for the ages of renal grafts that undergo late acute rejection