Bava Metzia 55: Worth a Perutah, Need a Fifth

2 mishnayot - Mishnah 1: Determining the differential to be considered exploitative, the amount to swear, and the amount to admit to owing money. Plus, 5 cases that depend on the equivalent of one perutah. Also, Mishnah 2: 5 halakhic categories for which "one-fifth" is relavant as an element of the halakhah.