Agroup of 500 transistors is known to contain one defective unit. what is the probability that a transistor selected at random from the lot is the bad one?

So, we can define probability as the following ratio: Number of successful outcomes over Number of total outcomes. Suppose we define an event to be the drawing of one transistor from the lot. If we want to calculate the probability that it is defective, a "successful" event would be drawing a defective transistor. Hence there is 1 successful event. But we can choose from 500 transistors, hence there are 500 total outcomes. Thus, the ratio that defines the probability P is: p=1/500. Calculating this, yields that p=0.002 or p=2%