It would be more useful if the numbers used at this point in the exercise were different from those used earlier in the exercise, and different from those used in the Tips. Otherwise, it just becomes a question of memory rather than understanding and applying the grammar point. This applies to most Duolingo exercises, which are too repetitive.
kuai is the equivalent of using "bucks" in English for dollars - If you are using the character 块 then that is the intended word... if you want to have it tranlated as yuan, then use the character 元。Both are legitimate names for the currency. What is maddening is that on some items the database contains both forms. A simple search and replace would make that a standard. Frankly, I think that if you use kuai it should be translated as kuai, and if yuan is used....