Carlos biked 139/8 miles on Saturday and 135/7 miles on Sunday. On which day did he ride further and by how much?

139/8 = 17.375 (this is rounded)
135/7 = 19.286 (this is rounded)

so he rode farther on Sunday

135/7 - 139/8 = 107/56 or 1 51/56 miles farther