We were told that Michelle did 81 crunches in 3 minutes. This means that the number of crunches that Michelle did per minute is
81/3 = 27 crunches per minute
her friend Zach did 104 crunches in 4 minutes. This means that the number of crunches that Zach did per minute is
104/4 = 26 crunches per minute
Therefore, Michelle did more crunches per minute