Percy probably made his estimate by rounding 0.58 per pound to 0.50, and rounding 2.03 for the price to 2.00. The 2.00/(0.50/lb) = 4 lb.
Percy's estimate is not unreasonable, but it is somewhat high (by more than 10%).
The cost, 0.58/lb, is closer to 0.6, a number that would cause Percy to estimate his purchase at 20/6 = 3 1/3 pounds.
When estimating, it is often useful to make an estimate of the error. This last estimate will be a little low, by about 3% due to the estimated cost and 1.5% due to the estimated total amount paid, for a total of about 4.5% low.
