To use in calculator:

type in the number you want to find the percent of. 6.5 billion (it has 8 zeros)

Hit the X or "Times" or "Multiply" button.

type in the percent you want, except move the decimal place left two places (if you dont' have enough numbers, add a zero). 5% would be 0.05

Push = or "Equals"
and as said, 325000000 is right.