Step 1:
The first step to converting 399.90921117878 to a fraction is to re-write 399.90921117878 in the form p/q where p and q are both positive integers. To start with, 399.90921117878 can be written as simply 399.90921117878/1 to technically be written as a fraction.
Step 2:
Next, we will count the number of fractional digits after the decimal point in 399.90921117878, which in this case is 11. For however many digits after the decimal point there are, we will multiply the numerator and denominator of 399.90921117878/1 each by 10 to the power of that many digits. So, in this case, we will multiply the numerator and denominator of 399.90921117878/1 each by 100000000000:
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 399.90921117878 as a fraction:
4399/11 / 1