Which of the following pair is the odd one out?

Study the following information carefully and answer the questions given below: Six persons are sitting in a row and an equal number of persons are facing north and south and they have different number of pens- 15, 16, 18, 19, 20, and 21 Z6 is sitting at one of the extreme ends. Persons sitting at extreme ends face the opposite direction and no person sits to the left of Z6. The one who is second to the right of Z4 is facing north and having 20 pens, but it is not Z6. Adjacent neighbours of Z2 are facing south direction. Two persons are sitting between Z6 and Z2. The difference between the number of pens with Z6 and Z5 is 4, which is equal to the difference between the number of pens with Z2 and Z4. Z6 has lesser pens than Z5. Z3 and Z1 are facing in opposite direction and Z1 is not adjacent to Z6 and Z5. Z1 does not have an even number of pens. The one who is immediate right of Z2 is not having 21 pens. Z1 does not sit adjacent to Z3.