4. A girl is standing on the first floor of a 24-story building, on the 13th, $16th$ and 24th floors of which her friends live. There are no stairs in the building, but there is an elevator that can only move 7 or 10 floors up or down. Can the girl visit all her friends by making no more than 10 elevator trips?
Solution. Yes, it can. For example, like this: 1 - 11 - 21 - 14 - 24 - 17 - 10 $20-13-6-16$