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Question: A machine produces bolts, of which 10% are defective. Find the probability that in a random sample of 400 bolts produced by the machine, between 35 and 45 will be defective. Assume a standard deviation of 6.


A ) 0.5640
B ) 0.6405
C ) 0.4605
D ) 0.4560

Steps to derive

1 The probability for a bolt to be defective, p = 10% = 110, q = 1 - p = 910and n = 400.

2 n·p = 400 × 110= 40 > 5
n·q = 400 × 910= 360 > 5
So, the use of normal approximation to the binomial distribution is appropriate.

3 Mean, μ = n·p = 400 × 110= 40

4 Standard Deviation, σ = npq = 400×110×910 = 6

5 Write the problem in probability notation: P (35 ≤ x ≤ 45), where x denotes the number of defective bolts chosen at random.

6 P (35 - 0.5 < x < 45 + 0.5) = P (34.5 < x < 45.5)
 [Rewrite the problem using the continuity correction factors.]

7 The corresponding area under the normal curve distribution is shown in the diagram.


8 The z1 value corresponding to x = 34.5 is given by, z1 = (x-μ)σ = (34.5-40)6 = - 5.56 = - 0.9167

9 Similarly, the z2 value corresponding to x = 45.5 is given by, z2 = (x-μ)σ = (45.5-40)6 = 5.56 = 0.9167

10 The area between z = 0 and z1 = - 0.9167 is 0.3203 by referring to the table for the probability z. Similarly the area between z = 0 and z2 = + 0.9167 is 0.3203.

11 From the diagram, the probability for z to be between z1 and z2 = the area for z1 + the area for z2 = 0.3203 + 0.3203 = 0.6406.

12 So, the probability that between 35 and 45 will be defective is 0.6405.



Hence the right answer is Option B

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