Direct object vs en

Question

J'ai mangé trois mangues.If ' trois mangues ' is underlined >> Je l'ai mangée.If only  ' mangues ' is underlined >> J'en ai mangé trois. Is this correct? Or do we always use en whether the number is underlined or not.

Accepted Answer

Esther, I am not sure of the context in the lesson/exercise, but either sentence can be used in conversations depending on situation.
The first sentence would be ‘je les ai mangées’ (plural) to be grammatically correct. It emphasises you have eaten ‘the mangoes’ - used in the context of knowing what ‘les’ represents in type and number (quantity). 
The 2nd sentence emphasises ‘how many’ of them - used in the case of knowing what ‘them’ is in type, but wanting or needing to specify the number ‘of them’.  
Your assumption about the significance of the underlining in a question or lesson seems likely correct.

Answer

Que diriez vous a "Je les ai mangée" ?

Answer

Luis, I am not sure what you are asking. Is your question seeking more explanation of the usage of the phrase per se, or are you questioning the grammar ? Both are correctly covered in the previous response, but can clarify if you have a specific question.

Answer

Thanks Maarten