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Every second meal I will have soup for my starter.
Every $5$th meal I will have pasta for my main course.
Every $3$rd meal I will have crumble for my dessert.

$2, 5$, and $3$ have no common factors, so I will have eaten $2\times5\times3=30$ school dinners (the Lowest Common Multiple) before I sit down to the same combination again.



This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.