Step 1:
The first step to converting 640.8398 to a fraction is to re-write 640.8398 in the form p/q where p and q are both positive integers. To start with, 640.8398 can be written as simply 640.8398/1 to technically be written as a fraction.
Step 2:
Next, we will count the number of fractional digits after the decimal point in 640.8398, which in this case is 4. For however many digits after the decimal point there are, we will multiply the numerator and denominator of 640.8398/1 each by 10 to the power of that many digits. So, in this case, we will multiply the numerator and denominator of 640.8398/1 each by 10000:
Step 3:
Now the last step is to simplify the fraction (if possible) by finding similar factors and cancelling them out, which leads to the following answer for 640.8398 as a fraction:
16021/25 / 1