* SD = standard deviation

The average (arithmetic mean) is equal to the sum of the n numbers divided by n:

*Average* = *sum / count* = (*a*_{1}+*a*_{2}+...+*a _{n}*) /

The average of 1,2,5 is:

*Average* = (1+2+5) / 3 = 2.667

The weighted average (*x*) is equal to the sum of the product of the weight (w_{i}) times the data number (x_{i}) divided by the sum of the weights:

Find the weighted average of class grades (with equal weight) 70,70,80,80,80,90:

Since the weight of all grades are equal, we can calculate these grades with simple average or we can cound how many times each grade apear and use weighted average.

2×70,3×80,1×90

*x* = (2×70+3×80+1×90) / (2+3+1) = 470 / 6 = 78.33333

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