Ciento and Cienta (100 and gender agreement)

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.

Here is a visual for you. 

I was going to write that the numbers 100-199 are too young to change in front of  girls.

The only time you make ciento change to cienta is for numbers over 200

as in doscientas cuarenta una galletas or doscientas una galletas.