What was the question for this implicit rationale, and corresponding answer?
There are 80 * 5 = 400 engines in total. 400 / 4 = 100 of the engines are defective. 400 - 100 = 300 of the engines are not defective.
 The answer: 300
The question: If one fourth of the engines are defective, and there are 5 batches of 80 engines each. How many engines are not defective?