If 20 out of 50 DVRs are said to be defective, the ones that are supposedly not defective would leave a total of 30 DVRs.
Defective - 20/50 (2/5 simplest form)
Not Defective - 30/50 (3/5 simplest form)
As shown directly above, there is a possible 30 out of 50 DVRs not defective.
Which means the probability rate is a 30/50 (3/5) chance. Percentage-wise, it'd be a 60% chance of choosing a non-defective DVR.
You're welcome.