AST SpaceMobile Validation Engineer interview questions

A farmer who owns 25 horses wants to determine which of them is the fastest, the second fastest, and the third fastest. What is the minimum number of races we must hold to obtain the following information: (1) A maximum of 5 horses can participate in each race. (2) We do not have a stopwatch. We cannot measure running times, only know who came first, second, etc. in each race. (3) The running speed of each horse is not ranked from race to race.