how to find standard deviation when given a percentage?

Question

I'm stuck on this question, can anyone help?

An electronics company's biggest seller is a talking toy. Of the toys produced, 4% have a defective voicebox. A quality control tech randomly collects 250 of the toys at the end of the production line for inspection. The standard deviation of the number of defective toys is ____?

Answer 1

Standard Deviation = sqrt(250*0.04*0.96) So about 3 in your case. Hope this helped!

Answer 2

The number of defective toys is a binomial $(250,0.04)$ random variable - each toy is treated as an independent Bernoulli trial where the "success" is voicebox defect. It suffices to use the standard formula for variance of a binomial variable.