Step 1:
The first step to converting 990.3988671878 to a fraction is to re-write 990.3988671878 in the form p/q where p and q are both positive integers. To start with, 990.3988671878 can be written as simply 990.3988671878/1 to technically be written as a fraction.
Step 2:
Next, we will count the number of fractional digits after the decimal point in 990.3988671878, which in this case is 10. For however many digits after the decimal point there are, we will multiply the numerator and denominator of 990.3988671878/1 each by 10 to the power of that many digits. So, in this case, we will multiply the numerator and denominator of 990.3988671878/1 each by 10000000000:
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 990.3988671878 as a fraction:
171339/173 / 1