The value of minus four minus twelve minus one ninth plus four minus one third

The value of minus four minus twelve minus one ninth plus four minus one third

-4-（-2/9-1/9）+(4-1/3)
=(-4+4)+(2/9+1/9-1/3)
=0

How to calculate the strength standard deviation of 38.4 36.1 38.2 45.8 47.9 37.8 43.5 34.3 37.0 39.5 45.0 46.2 42.0 45.7 35.7
Detailed explanation, thank you

First find the average: x = 38.4 + 36.1 + 38.2 + 45.8 +... + 45.7 + 35.7 = 6131
Then calculate the standard deviation: {[(38.4-x) ^ 2 + (36.1-x) ^ 2 + (38.2-x) ^ 2 +... + (35.7-x) ^ 2] / 15} ^ 0.5 = 4.3426

Calculation: 8 / 17 × 7 / 10 + 7 / 10 × 9 / 17 + 3 / 10 =? 0.84 × [7 / 10 × (1 / 2 × 7 / 2)]

8/17×7/10+7/10×9/17+3/10
=(8/17+9/17)×7/10+3/10
=1×7/10+3/10
=7/10+3/10
=1
0.84÷【7/10×（1/2×7/2）】
=0.84÷【7/10×7/4】
=84/100÷49/40
=21/25×10/49
=3/5×2/7
=6/35