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jmc

number theory intermediate

Problem

When we say that Ray is climbing up the stairs at a time, we mean that he starts on the floor (step ) then jumps to step and then to and so on until the number of steps to the top is less than . Ray climbs up a flight of stairs of steps in two ways. When he does it steps at a time, there are steps left at the top. When he does it steps at a time, there are steps left at the top. What is the smallest possible value of that is greater than ?
Solution
The given information translates to the congruences From the first congruence we obtain that for some integer Combining this result with the second congruence, we have Therefore, So, for some integer Substituting for , we have The smallest such greater than is .
Final answer
27