How do you write out 6.4 million in numbers including the zeros?

To write out 6.4 million in numbers including the zeros, you would write it as 6,400,000. This is because the number 6.4 million consists of 6 million (6,000,000) and an additional 400,000 (400,000). Therefore, when written out in numerical form, it becomes 6,400,000.