Translation:You only need to pay five percent interest.
I concede that one expression is more common, but Lisa939448 is right. Her translation is perfectly acceptable as an English sentence (especially because I translated the sentence the same way). Could Duo be a bit more flexible since the object under consideration here is learning Swedish, not debating which forms of English diction are more popular?
"Undergraduate students can expect to pay 4.45 percent in interest on new Stafford loans, instead of the current 3.76 percent."--Danielle Douglas-Gabriel, "Say Goodbye to Low Interest Rates on Student Loans, at least for Now," Washington Post, May 11, 2017.
Well, I'm not a contributor any longer, so I don't have final say. I'm not convinced the usage is right, although I do appreciate your comment.
I will say though that incorrect English will never be consciously accepted throughout the course, so if the course contributors consider the usage incorrect rather than more rare, it will not be accepted. That's just to clarify policy in general, not meant as a comment on the specific sentence.