Maths Olympiad Prep

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Number theory Difficulty 4.7 AIME Find the answer

Find the rightmost non-zero digit of the expansion of (20)(13!).

A number or a short expression. Spacing and $ signs are ignored.

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)(10 \times 2)(13 \times 12 \times 11 \times 10 \times 9 \times 8 \times 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1)=\left(10^{3}\right)(2 \times 13 \times 12 \times 11 \times 9 \times 8 \times 7 \times 6 \times 4 \times 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: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.