If 9 bags of oats will last 8 horses for 7 days, how long will 6 bags of oats last 5 horses?

Since 9 bags last 8 horses 7 days, 6 bags will last 8 horses 2/3 as long.

2/3 x 9 = 6 bags will last 8 horses 2/3 x 7 = 14/3 = 2 2/3 days.

But 6 bags will last 5 horses 8/5 times longer.

8/5 x 14/3 = 112/15 = 7 7/15 days (roughly a week).