What does "I normally add 15 to 20%." mean in English for Spanish?

In English for Spanish, "I normally add 15 to 20%." means:

Yo normalmente añado el 15 o 20%.

Listen to "I normally add 15 to 20%.": (If you have an HTML5 enabled browser, you can listen to the native audio below)

This is a phrase that is used in the GamesForLanguage English for Spanish Language Game in the following scenes:

• English for Spanish 3, Level 3, Scene 3