Gender agreement and Spanish numbers are only an issue when the numbers are over 200.
If the number is between 101 and 199, the ciento part does not ever agree with a feminine noun. That is to say that you do not put an A at the end of the word ciento to make it feminine. If the number has an un (1) in the number ( 101, 121,131 etc ), then the un part does get an A at the end... if the noun is feminine.
Example:
ciento cuarenta una galletas or ciento una galletas. Notice that the ciento part is not feminine even though the noun it is modifying is.
Example:
ciento cuarenta una galletas or ciento una galletas. Notice that the ciento part is not feminine even though the noun it is modifying is.
Here is a visual for you.
The only time you make ciento change to cienta is for numbers over 200
as in doscientas cuarenta una galletas or doscientas una galletas.