Four number-pairs have been given, out of which three are alike in some manner and one is different. Select the number-pair that is different.

15 : 197 25 : 167 27 : 181 21 : 139

15 : 197

The logic is : (Ist number × 7) - 8 = second number 15 : 197 = (15 × 7) - 8 = 97 but 197 is given (ODD) 25 : 167 = (25 × 7) - 8 = 167 27 : 181 = (27 × 7) - 8 = 181 21 : 139 = (21 × 7) - 8 = 139