The variance quotient is the weighted squared-deviation numerator over total weight. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The numerator is the sum of weighted squared terms
The three contributions are 4, 0, and 4. Their sum is 8.
Nσ=4+0+4=8
The denominator is total weight
The weights are 1, 2, and 1. Their total is 4.
W=1+2+1=4
Variance is numerator over total weight
Variance divides numerator 8 by total weight 4, giving 2.