Step 1:
The first step to converting 666.9807421878 to a fraction is to re-write 666.9807421878 in the form p/q where p and q are both positive integers. To start with, 666.9807421878 can be written as simply 666.9807421878/1 to technically be written as a fraction.
Step 2:
Next, we will count the number of fractional digits after the decimal point in 666.9807421878, which in this case is 10. For however many digits after the decimal point there are, we will multiply the numerator and denominator of 666.9807421878/1 each by 10 to the power of that many digits. So, in this case, we will multiply the numerator and denominator of 666.9807421878/1 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 666.9807421878 as a fraction:
34016/51 / 1