Number theoryDifficulty 4.2AIMEFind the answerUnited States
Problem:
Find the rightmost non-zero digit of the expansion of (20)(13!).
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Solution:
We can rewrite this as (10×2)(13×12×11×10×9×8×7×6×5×4×3×2×1)=(103)(2×13×12×11×9×8×7×6×4×3). Multiplying together the units digits for the terms not equal to 10 reveals that the rightmost non-zero digit is 6.
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Source: MathNet,
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