AMC 10 Step by Step
2013 AMC 10BProblem 25P21-25~12 minPrint

Bernardo chooses a three-digit positive integer NN and writes both its base-5 and base-6 representations on a blackboard. Later LeRoy sees the two numbers Bernardo has written. Treating the two numbers as base-10 integers, he adds them to obtain an integer SS . For example, if N=749N = 749 , Bernardo writes the numbers 10,44410,444 and 3,2453,245 , and LeRoy obtains the sum S=13,689S = 13,689 . For how many choices of NN are the two rightmost digits of SS , in order, the same as those of 2N2N ?

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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.