Solution:

Here `n = 5`, Mean X `= 20`
The sum of `5` observation is = X × n `= 20 × 5 = 100`
After adding one new observation, the new Mean X `= 25`,
the sum of `6` observation = new X × n `= 25 × 6 = 150`
`:.` the new observation `= 150  100 = 50`.


