A reasonable estimate is within an error band that is acceptable for the problem at hand. An unreasonable estimate is not. Examples It is (probably) unreasonable to estimate the value of 1 + 1 as 200. It is (probably) unreasonable to estimate the value of .0034504 - .0034501 as .3 It may be reasonable to estimate the sum of 4 + 5 as 10. It may be reasonable to estimate the sum of 40 + 30 as 0 (when rounding to the nearest hundred, for example). It may also be reasonable to estimate the same sum as 100. The amount of error that is acceptable may be determined, in part, by the amount of error in the numbers given with the problem. For example, when asked to estimate the number of heartbeats in an average lifetime, one might guess what the average heart rate is and what the average lifetime is. The normal resting heart rate of a person varies with age and fitness over a range of perhaps 2:1, so any estimated value can carry considerable error. Likewise, we have pretty good statistics on median age at death, but very little information is available on average age at death. A guess will carry significantly more error. Using these erroneous values in a calculation will yield a result that will be considered reasonable by someone who agrees with the numbers and the method you used. It may be considered unreasonable by a person who does not agree.