If a can of dog food can feed 5 dogs for 4 days, how long would it feed 8 dogs?

This must either be a large can of dog feed or some mighty small dogs :):)

Anyway, one can contains 5\cdot 4=20 dog-days worth of food.

Intuitively, we know the number of days the can feeds 8 dogs is going to be less than 4 because there are more dogs.

Since 5\cdot 4=20, then 8\cdot x=20

What is x?