"Forty, fifty, sixty"
Translation:마흔, 쉰, 예순
There's two different number systems. The one Duolingo is using in this question starts with 하나, 둘, 셋 while the one you're referring to starts with 일, 이, 삼 (based on Chinese numbers). I personally have never heard of anything beyond 마흔 and the only place I've heard of it being used was in reference to someone's age.