脑筋急转弯

question:

Given 25 horses, find the best 3 horses with minimum number of races. Each race can have only 5 horses. You don't have a timer.

answer:

7 races

 

questions:

there are 81 horses. you need to select the first best 6 horses. Each time only 9 horses can participate. how many minimum no. of races you require