How do you write 2.15 billion?

To write 2.15 billion in numerical form, you would express it as 2,150,000,000. This is done by placing the digits according to the value of each position, with the '2' in the billions place, the '1' in the hundreds of millions, the '5' in the tens of millions, and the '0's filling in the remaining places. In scientific notation, it can also be represented as 2.15 × 10^9.