Step 1:
The first step to converting 121.78900520172 to a fraction is to re-write 121.78900520172 in the form p/q where p and q are both positive integers. To start with, 121.78900520172 can be written as simply 121.78900520172/1 to technically be written as a fraction.
Step 2:
Next, we will count the number of fractional digits after the decimal point in 121.78900520172, which in this case is 11. For however many digits after the decimal point there are, we will multiply the numerator and denominator of 121.78900520172/1 each by 10 to the power of that many digits. So, in this case, we will multiply the numerator and denominator of 121.78900520172/1 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 121.78900520172 as a fraction:
13275/109 / 1