"How much does he earn?"
Translation:Combien il gagne ?
As you can see (gagne) ends with a vowel (e) ... and (il) begins with a vowel too (i) ... and as in French two vowels in separate words can't follow each other ... they have to be separated with this (t) letter ... That's why it comes in the first solution and doesn't come in the second one ....
First, "don't we say in language X" is rarely a good argument when leaning language Y.
Second, the literal English translation would be "how much he (does) make(s)." Just like your French proposal it can only be used in a subordinate question: I know how much he makes = je sais combien il fait.
I went with "mérite" instead of "gagne" and got it wrong. To me, "earn" is halfway between "deserve" and "win," and choosing between the two was difficult. I'm guessing that from the context (where it sounds like one is asking about wages), the verb gagner is more correct (by convention), but if someone were to say after going on a long jog, "I have earned this ice cream sundae," then using the verb mériter in the translation would be right.
When you see a new word, duolingo highlights it for you. If it does not highlight a new word, it believes you've already mastered it. If that is not the case, you may also hover your mouse cursor over the word to see its french-english translation, although at the cost of familiarity, which the system will note.