What happens to the mean of a set of numbers when a zero is added to the set?

The relative value of  the adding zero  with respect to the
existing average counts

If zero is greater than  the former average
, then by adding a zero the average creases.

If the former average
itsef is zero ,  and now a zero is added then the average remains
unaltered.

If the former average is positive , then zero is less
than the former average. So adding a zero decreases the
average.

Example:

a)

The
temperature of a place on two days are -5  degreeeC and -7 degree. So the average of 2 days =
(-5-7)/2 = -6 degree.

The temperature of the place on the third day
is zero. So the average of 3 days = (-5-7+0 )/3 = -4. So the average temperature has increased
from -6 deg C. to -4 deg C.

b)

The
temperature of the place on 3 days are : -2 deg C , 2 deg C and zero deg C. Then the average for
first two days is (-2+2)/2 = 0. And the average for the 3 days = (-2+2+0)/3 = 0. So the average
temperature remianed same.

c)

If the
temperatures of the city  for the 3 dates is : 1 deg C, 3 deg C and zero deg C, then the average
for the  first 2 days is (1+3)/2 = 2 C. The average for the 3 days = (1+3+0)/3 = (4/3) deg C
.