Question

Ten of the 50 digital video recorders in an inventory are known to be defective. what is the probability you randomly select an item that is not defective?

Answer 1

10 - are defective
50 - 10 = 40   -  aren't defective

$\overline{\overline{A}}= 40 \\ \\ \overline{\overline{\Omega}}=50 \\ \\ P(A)=\dfrac{\overline{\overline{A}}}{\overline{\overline{\Omega}}}=\dfrac{40}{50}=\dfrac{80}{100}=80\%$

The probablity is 80%.