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Rolling a d10 until the lcm passes 2000

Question

You roll a fair ten-sided die (faces 11 to 1010) repeatedly and track the least common multiple of all the values rolled so far. Let the expected number of rolls until this lcm first exceeds 20002000 be pq\tfrac{p}{q} in lowest terms. Compute p+qp+q.

Short answer

119119. The lcm of 1,…,101,\dots,10 is 25202520, and every proper divisor of it is at most 12601260, so you need the full 25202520: a 77, an 88, a 99 and a multiple of 55 (55 or 1010). A coupon-collector style calculation (or a small Markov chain over lcm values) gives 1136\tfrac{113}{6} expected rolls.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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