un fils modele and une fille modele (sorry on my laptop so can't add accents)

Sharon, 

As per the lesson you linked, the adjective modèle is the same form in masculine and feminine because it ends in a mute ‘ e’. 

With any adjective that ends in a terminal mute ‘ e ‘ in its ‘default’ form, such as ‘ modèle ‘, an additional ‘ e ‘ is not added for the feminine form, and the masculine and feminine forms are identical to each other, in both singular and plural forms ( with the addition of an s in the latter of course ). 

By extension from the ‘rule’ applying to adjectives ending in a mute ‘ e ‘, adjectives ending with ‘ é ‘ in default form, will take an added terminal ‘ e ‘ in feminine form. 

Just the way it is. 

Adjectives ending with mute -e don't change in the feminine in French

https://www.wordreference.com/fren/mod%C3%A8le

Thanks!!! Very helpful. I am a long term student (senior citizen) and find these tips extremely useful.