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Number theory Difficulty 2.5 Junior Find the answer

What is the tens digit of the smallest six-digit positive integer that is divisible by each of 10,11,12,13,1410,11,12,13,14, and 15?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Among the list 10,11,12,13,14,1510,11,12,13,14,15, the integers 11 and 13 are prime. Also, 10=2×510=2 \times 5 and 12=2×2×312=2 \times 2 \times 3 and 14=2×714=2 \times 7 and 15=3×515=3 \times 5. For an integer NN to be divisible by each of these six integers, NN must include at least two factors of 2 and one factor each of 3,5,7,11,133,5,7,11,13. Note that 22×3×5×7×11×13=600602^{2} \times 3 \times 5 \times 7 \times 11 \times 13=60060. (This is the least common multiple of 10,11,12,13,14,1510,11,12,13,14,15.) To find the smallest six-digit positive integer that is divisible by each of 10,11,12,13,14,1510,11,12,13,14,15, we can find the smallest six-digit positive integer that is a multiple of 60060. Note that 1×60060=600601 \times 60060=60060 and that 2×60060=1201202 \times 60060=120120. Therefore, the smallest six-digit positive integer that is divisible by each of 10,11,12,13,14,1510,11,12,13,14,15 is 120120. The tens digit of this number is 2.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.