Question

In a manufacturing plant, the probability of making a defective bolt is 0.1. The mean and standard deviation of defective bolts in a total of 900 bolts are respectively

• A. 90 and 9
• B. 9 and 90
• C. 81 and 90
• D. 9 and 81

Answer 1

The correct option is A 90 and 9

Using Binomial distribution
Mean = np = 900 × 0.1
$\mathrm{mean}=90$

Variance = npq
= 900 × 0.1 × 0.9 = 81
Standard deviation,
$\sigma =\sqrt{\mathrm{variance}}=\sqrt{81}$

$\sigma =9$