Number theory · Linear diophantine equation · Divisibility · Coprimality · Parity
A competition paper had \(20\) problems. Each problem answered correctly was worth \(8\) points, each problem answered incorrectly cost \(5\) points, and a problem left blank scored \(0\). Tine handed in his paper and learned that his total was \(13\) points. How many of the problems did he attempt?
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Slovenian High School Mathematics Competition for Vega Awards (MaSSA), izbirno (selection) round 2007, 1. letnik, category A, problem 2. Organized by DMFA Slovenije (Society of Mathematicians, Physicists and Astronomers of Slovenia). Source