If one fourth of the engines are defective, and there are 5 batches of 80 engines each. How many engines are not defective?
There are 80*5 = 
<gadget id="calculator">80*5</gadget>
<output>400</output>
400 engines in total.
400/4 = 
<gadget id="calculator">400/4</gadget>
<output>100</output>
100 of the engines are defective.
400-100 = 
<gadget id="calculator">400-100</gadget>
<output>300</output>
300 of the engines are not defective.

<result>300</result>