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2016•The Mathematical GazetteRequires access

Will my numbers add up correctly if I round them?

David Hopkins

Open publisher page 1 citations

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.

About this research paper

What this paper is about

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.

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OpenAlex reports 1 citations for this work. Citation counts describe recorded attention and do not establish research quality.

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Available 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.

Key concepts: Rounding, Decimal, Arithmetic, Computer science, Mathematical economics, Mathematics, Calculus (dental), Combinatorics

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