You are given 12 identical looking marbles, with 1 weighting less than the rest. Given a balance scale, how would you go about identifying the marble that weights less in the least amount of use of the balance scale?

There are 25 horses and 5 tracks. Horses run at their original speed, regardless of how many races they run. There are no stopwatches or other equipment, and the only way to determine the order is ranking in a specific race. What is the least number of races needed to determine the top 3 horses?