Discussion
4 friends visit a church on every 13th, 15th, 17th and 18th day of the month. If all of them met at the church on a Monday, then on which day will they meet next?
Answer: D
They will next meet on a Thursday.
Reading the visits as every 13, 15, 17 and 18 days, all four are together again after the LCM of those numbers.
\[ \begin{aligned} \text{LCM}(13, 15, 17, 18) &= 13 \times 17 \times 90 \\ &= 19890 \text{ days} \\ 19890 &= 7 \times 2841 + 3 \end{aligned} \]
19890 days is 2841 complete weeks plus 3 days, so the meeting falls 3 days after a Monday, on Thursday. Note that 15 and 18 share a factor 3, so the LCM uses 90, not \(15 \times 18 = 270\). Multiplying all four numbers gives 59670 days, which lands on a Wednesday, not one of the options but a sign of the slip.
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