OE

Standard deviation signifies, deviation of the terms from its mean value
That means, more deviated are the terms from its mean, more is SD.
To keep standard deviation minimum, we add a term which is closest to the
mean and a number which is closest to mean is mean itself.
Consider the set of terms 10, 20, 30, 40, 50.
Here mean is 30. (As the terms are in AP, mean and median is same).
Now, to have minimum standard deviation after adding a new term to the above set,
the new term should be equal to the mean.
i.e. if we add 30 to (10, 20, 30, 40, 50), then we can see that 30 is not at all deviated
from the mean.
As standard deviation signifies ‘deviation from mean’, we did not let the term deviate
from the mean.
Ans. (30)