250 billion is 250,000,000,000
So, you can write it in scientific notation as:
[tex]2.5\times10^{11}[/tex]Because you need to count the spaces from the decimal point to the last zero and elevated 10 to this power. So 250,000,000,000 is equivalent to:
2 . 5 0 0 0 0 0 0 0 0 0 0 x 10^11
1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th