In a batch of 960 calculators, 8 were found to be defective. What is the probability that a calculator chosen at random will be defective? Write the probability as a percent. Round to the nearest tenth of a percent if necessary

This is a case of empirical (observed) probability.  Historically speaking, 8 out of 960 calculators were found to be defective.  

Thus P(defective calculator) = 8/960, or 0.008333 ...

As a percent, this is 0.8%.