Solution:
Let S be the sum of the 21 different numbers.
Then (S - n) / 20 is the average of the other 20 numbers in the list.
Hence,

n = 4 ((S - n) / 20) = (S - n) / 5.

Solving for n:

5n + n = S
6n = S
n = S / 6.

Therefore, n is 1 / 6 of the sum (S) of the 21 numbers in the list. ANS: B) 1/6