I guessed right, but I agree with you, Google Translate pronounces both "ajoute" and "ajoutent" the exact same way so I'm guessing Duolingo does the same, and with Duolingo pronouncing "Il" and "Ils" in a very similar way (if not identical), it gets really hard to discern. It would really help if Duolingo added some more emphasis on the pronunciation of the plural on "Ils".
Try finding an online dictionary with pronunciation or typing them all into google translate and hitting the audio button. That should help you differentiate il and elle. However, in the Paris dialect, il vs. ils and elle vs. elles are indistinguishable, except when there is a liasion (for example, if this sentence used ils there would be a liasion.)