Given that The image of a dust mite from a scanning electron microscope is
millimetres wide. And that image is
times life size. Now we have to find how many millimetres wide is the dust mite. To find that we just need to divide the the width of dust mite by the scale value of microscope. I mean divide
millimetre by

So the answer is given by
Width of the dust mite

Hence final answer is 0.3 milimetre.