We can start by setting out what we know simply.
Cow produces 5 liters a day.
The cow can make 200% more.
200% of 5, is 10.
If he makes 200% more, he will make 10 liters more.
He already makes 5.
10+5 is what he will make.
FINAL ANSWER:
"If the cow's diet is improved, the cow will produce 15 liters of milk in a day."
