Ratter....
I 100% get where you're coming from, but you're wrong. It is the whole point of the riddle to mislead but I don't think anyone has hit the nail on the head with the correct and clear explanation yet...
What you're assuming is that because it takes 5 dogs, 5 minutes that that equates to 1 minute PER DOG. When it doesn't. If you think about the logistics and imagine that there are 5 dogs and 5 rats locked in a room for 300 seconds., the way you're suggesting it would mean that a Dog A has between 0-60 seconds to kill a rat whilst Dogs B,C,D & E stand around and watch, then Dog B takes between 61-120 seconds to get his rat, and so on until the 300 seconds is up and they've taken a minute each to each kill one rat. (That's where you're 1 dog, 1 minute, 1 rat thing comes in!)
What the riddle actually suggests is that it takes ALL 5 DOGS (i.e. all actually running around ratting) the whole 300 seconds to kill all the rats. Which actually means Dog A is chasing Rat A for the whole 5 minutes, whilst at the same time Dog B is chasing Rat B for the whole 5 minutes etc etc. Meaning it actually takes 1 dog, 5 minutes to catch the rat.
Now imagine Dog A has killed Rat A in his 5 minute allocation, he'd then move on to Rat F (with Dog's B,C,D & E, busy taking care of Rat's B,C,D & E), then it would take all 5 of the Dogs, 100 minutes to get the 100 rats.
Hence the answer = 5.