If you ask old Ursula how many cats she has at home, she answers sadly: "Four-fifths of my cats plus four-fifths of a cat." How many cats does this add up to?

She has 4 cats......

(4/5) of them =  (4/5)4 = 16/5  = 3.2 cats

And 4/5 of 1 cat = .8 of a cat

So.....3.2 cats + .8 of another = 4

Algebraically.....let N be the number of cats......

Then.....

(4/5)N + 4/5  = N

4/5  = (1/5)N

And it's obvious that N = 4......