So, the dog eats 5 cups a day and 1 pound of dog food is 10.5 cups.
A 50 lb bag is $22.50.
Since you're asking how much it would cost to feed it a day we need to find out how much 1 pound of food costs.
22.50 / 50 = 0.45
So, it's 0.45 for 1 lb. But 1 lb is 10.5 cups, not 5.
0.45 / 5 = 0.09
So, I believe it would cost 0.09 each day to feed the dog.