Final answer:
The greatest common factor of 20y^9 + 5y^6 is 5y^6.
Step-by-step explanation:
To find the greatest common factor (GCF) of 20y^9 + 5y^6, we need to determine the highest power of each variable that divides each term evenly. In this case, the highest power of y that can be divided evenly is y^6. The GCF of the two terms is therefore 5y^6.