Well, let's take a moment to appreciate the beauty of numbers. If we gently divide 400 by 25, we find that it can go in 16 times. Remember, there's no mistakes in math, just happy little accidents!
Well, 4 divided by 16 is 1/4, so 16 into 400 billion it must be
100 billion times that. That makes it 25 billion. Or if you like,
400 divided by 16 is 25, leading to the same answer.