The unit’s place digit of 129121+7867 = Unit’s place digit of 129121 + Unit’s place digit of 7867 = Unit’s place digit of 9121 + Unit’s place digit of 867 = Unit’s place digit of 91 + Unit’s place digit of 83 (Since the order of cycles of 9 and 8 are 2 and 4 respectively) = Unit’s place digit of (9 + 2) = 1