Every second meal I will have soup for my starter.
Every $5$th meal I will have meat 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 before I sit down to the same combination.
This problem is taken from the UKMT Mathematical Challenges.