This is a classic
inclusion-exclusion problem on LCMs — and the key phrase to watch is "exactly two."
Step 1: Identify the visit cycles.Starting Jan 1, 2020 (day 0), over a full year (2020 is a leap year, so 366 days):
- Ryan visits every 3rd day: days 3, 6, 9, ...
- Jason visits every 4th day: days 4, 8, 12, ...
- Sharon visits every 5th day: days 5, 10, 15, ...
Step 2: Count individual visits.- Ryan: ⌊366/3⌋ = 122 days
- Jason: ⌊366/4⌋ = 91 days
- Sharon: ⌊366/5⌋ = 73 days
Step 3: Count pairwise overlaps (days when at least two visit together).- Ryan AND Jason: LCM(3,4) = 12 → ⌊366/12⌋ = 30 days
- Ryan AND Sharon: LCM(3,5) = 15 → ⌊366/15⌋ = 24 days
- Jason AND Sharon: LCM(4,5) = 20 → ⌊366/20⌋ = 18 days
Step 4: Count days when ALL three visit.LCM(3, 4, 5) = 60 → ⌊366/60⌋ = 6 days
Step 5: Apply the "exactly two" formula.Each day when all three meet is counted once in each of the three pairwise counts. We want to remove those days entirely from our "exactly two" count:
Exactly two = (30 + 24 + 18) − 3 × 6 = 72 − 18 =
54 daysCommon trap: The biggest mistake is computing "at least two" (which is 72) instead of "exactly two" (54). When you see "exactly" in a counting problem, you must always subtract the triple overlap. Students who skip Step 5 land on answer choice E (72) — which is there on purpose as a trap answer.
Takeaway: For any "exactly k out of n" overlap problem, always start with inclusion-exclusion and be precise about stripping out higher-order overlaps. Writing out the Venn diagram for 30 seconds can save you from a careless error.
Answer: D (54 days)---
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