Six horses eat 12 bales of hay in 12 hours. At the same rate, how many hours will 36 bales of hay last 12 horses?
From the original rate, we note that 12 horses will eat 12 bales of hay in 6 hours. (Double the number of horses, half the time)  Therefore, 12 horses will eat 36 bales of hay in $\boxed{18\text{ hours}}$. (Triple the amount of hay, triple the time)