Question

if there are 400 billion birds in the world and 5 billion die each year and 395 million are left but 10 billion birds are made each year how many birds are there really

Answer 1

You already said the answer. There are 400 billion birds in the world. ("if there are 400 billion birds in the world...")

Also, if you wanted an approximate calculation of everything, you would have to tell us how many years there are. Is this a trick question?