Will my numbers add up correctly if I round them?. Issue 549 (17th October 2016)

• Record Type: Journal Article
• Authors: Hopkins, David
• Is Part Of: Mathematical gazette. Volume 100:Issue 549(2016)
• Page Start: 396
• Page End: 409
• Publication Date: 2016-10-17
• Subjects: Mathematics -- Periodicals; Mathématique; Mathematik; Mathematics; Periodicals; Ressource Internet (Descripteur de forme); Périodique électronique (Descripteur de forme); Zeitschrift; Online-Publikation; 510
• DOI: 10.1017/mag.2016.104
• Languages: English
• ISSNs: 0025-5572

Abstract

Abstract : I recently wrote a solution to a student problem. The answers were the probabilities for four possible outcomes. As a check on the calculations, I wanted to say 'As expected, these probabilities add up to 1.' However, they didn't! Because of rounding, the probabilities (which I was quoting to four decimal places) only added up to 0.9999. To rescue the situation I tried rounding to 3DP or 5DP instead, but these didn't add up correctly either.