Duolingo is a platform on which users can learn over 40 languages, such as English or Spanish, free of charge.
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The bulk (~ 90%) of the revenue that Duolingo generates comes from a premium subscription called Plus.
Users on the free tier occasionally see ads, which appear within the progress board, between lessons, and within your profile.
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In 2014, Duolingo launched online language proficiency tests that cost $49 per taken exam.
DUOLINGO ENGLISH TEST WEBSITE