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Answer:
P [ 55 ≤ X ≤ 70 ] = 23.55 %
Step-by-step explanation:
We know:
μ₀ = mean of population σ = standard deviation
We begin for calculating the probability of X ≤ 55 so
Step 1:
μ₀ = 50 σ = 7
×₁ = ( μ - μ₀ ) ÷ σ ⇒ ×₁ = (55-50) ÷ 7 ⇒ ×₁ = 5 ÷ 7 ×₁ = 0.7143
We look z table and we have to interpolate
value (closest smaller Than ×₁ 0.71 and closest bigger than ×₁ 0.72
The associated area for these point are: 0.7611 and 0.7642
Taking diferences and by rule of three
0.01 ⇒ 0.0031
0.0042 ⇒ Δ Δ = 0.0013 and Area for ×₁ = 0.7624
That is the probability of X ≤ 55
Now we have to find the probability of X ≤ 70
We procede as in step 1
μ₀ = 50 σ = 7
×₂ = ( μ - μ₀ ) ÷ σ ⇒ ×₂ = (70-50) ÷ 7 ⇒ ×₂ = 20 ÷ 7 ×₂ = 2.8571
We look z table and we have to interpolate
for 2.85 ⇒ 0.9978 (area) 2.8571 - 2.85 = 0.007
for 2.86 ⇒ 0.9979 (area)
Therefore by rule of three we have
0.01 ⇒ 0.0001
0.007 ⇒ Δ Δ = 0.00007
And area for ×₂ = 2.8571 is equal to 0.99787 and the probability of
X ≤ 0.99787 or 99.79
Now if we look the annex drawing (the area we are looking for is enclose for the values x₁ and ×₂ (pink area) and is the diference between these two areas
So P [ 55 ≤ X ≤ 70 ] is 99.79 % - 76.24 % = 23.55
