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Ruff Numbers

 Published on Saturday, 20th November 2021, 07:00 pm and solved by 310
Difficulty: Level 29 [73%]

Problem 773

Let $S_k$ be the set containing $2$ and $5$ and the first $k$ primes that end in $7$. For example, $S_3 = \{2,5,7,17,37\}$.

Define a $k$-Ruff number to be one that is not divisible by any element in $S_k$.

If $N_k$ is the product of the numbers in $S_k$ then define $F(k)$ to be the sum of all $k$-Ruff numbers less than $N_k$ that have last digit $7$. You are given $F(3) = 76101452$.

Find $F(97)$, give your answer modulo $1\,000\,000\,007$.



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