Question

A manufacturer tested $1000$ cell phones at random and found that $25$ of them were defective. If a cell phone is selected at random, what is the probability that the selected cellphone is a defective one

Answer 1

Total number of cell phones tested $=1000$ i.e., $n=1000$
Let $E$ be the event of selecting a defective cell phone.
$n\left(E\right)=25$ i.e., $m=25$
$P\left(E\right)=\frac{\text{Number of defective cellphones}}{\text{Total number of cellphones tested}}$
$=\frac{m}{n}=\frac{25}{1000}=\frac{1}{40}$