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P26 [Combinatorics] - Turkish NMO 1st Round - 2001

Source:

April 23, 2014

Problem Statement

Berk tries to guess the two-digit number that Ayca picks. After each guess, Ayca gives a hint indicating the number of digits which match the number picked. If Berk can guarantee to guess Ayca's number in nn guesses, what is the smallest possible value of nn?
<spanclass=latexbold>(A)</span> 9<spanclass=latexbold>(B)</span> 10<spanclass=latexbold>(C)</span> 11<spanclass=latexbold>(D)</span> 15<spanclass=latexbold>(E)</span> 20 <span class='latex-bold'>(A)</span>\ 9 \qquad<span class='latex-bold'>(B)</span>\ 10 \qquad<span class='latex-bold'>(C)</span>\ 11 \qquad<span class='latex-bold'>(D)</span>\ 15 \qquad<span class='latex-bold'>(E)</span>\ 20