If arithmetic mean of 20 numbers x1, x2, ....., x20 is M, then the value of ∑(i=1 to 20) (xi - M) / 20…

Since the arithmetic mean of $x_1, x_2, \dots, x_{20}$ is $M$, we have $\sum_{i=1}^{20} x_i = 20M$. The algebraic sum of deviations from the arithmetic mean is always zero: $\frac{1}{20}\sum_{i=1}^{20} (x_i - M) = \frac{20M - 20M}{20} = 0$.