Answer
$$ (a) \ 608.9\ billion \\ (b) \ 42.387 \ million \\ \ (c) \ 85.2 \ thousand$$
Work Step by Step
To write each number as a number with a decimal point, we need to identify the location of all of the digits. We know that 'thousand" is the 4th digit, "million" is the 7th digit, and that "billion" is the 10th digit. All values other than the largest value (thousands, millions, billions, etc) are written as decimals. Doing this, we find:
$$ (a) \ 608.9\ billion \\ (b) \ 42.387 \ million \\ \ (c) \ 85.2 \ thousand$$
