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Repunit-style numbers whose product equals their sum

Question

Find two real numbers xx and yy, each written using only the digit 11 in decimal (a decimal point is allowed), such that xy=x+yxy=x+y. What is the common value of xyxy?

Short answer

12.112.1. The condition rearranges to x=yy−1x=\tfrac{y}{y-1}. Taking y=11y=11 gives x=1110=1.1x=\tfrac{11}{10}=1.1, and indeed 11×1.1=11+1.1=12.111\times1.1=11+1.1=12.1.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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