Trick photography asks: “What size is the missing wrench?”


Technically the image isn’t trick photography but the gap in positioning makes you think the size of the missing wrench must be between \frac{1}{2} and \frac{9}{16} of an inch. It may be a form of trickery but could a student argue the missing wrench size is  \frac{17}{32}? How would they justify the break in the pattern?

wrench fractions

One of the wrenches is indeed missing. Would students think to mentally close the gap and consider the possibility that the first or last wrench in the sequence is missing? What would be those sizes?

The idea is to let students explore common denominators and the patterns they discover in this task.

I’ll let you know how it goes.

5 thoughts on “Trick photography asks: “What size is the missing wrench?”

    1. I think those types of weights are consistently measured in lbs or kilos. But you have me thinking about other “things” that may be out of place–and not necessarily in units of measure. Yet I’m not quite sure what those mathematically puzzling situations would be.

  1. I’m glad I came across your post this evening. I’m going to be using the picture in your post with my fourth and fifth grade crew next week. The picture not only looks at patterns, but the question you asked will lead students to look for common denominators. I find pictures to be a unique and engaging way to introduce mathematical concepts. Thanks for sharing, Mary.

    1. Hi Matt! In my 7th grade standard classes I have a small crop of students who are still baffled by fractions. They may never get it, but it doesn’t hurt to keep trying! Some may think it’s a low level task, but my husband and I were arguing about it on a dinner napkin at Baker’s Square! Thanks for stopping by.

  2. Wrenches are an excellent way to teach powers of two fractions. I do use this technique. Be aware, though, 11/32″ is a normal size (fits No.8 machine screw nuts) while the other larger “32nds” sizes are obsolete or at least very uncommon. 13, 15, 19, 25 and 31 thirty-seconds used to be standards.
    Also, do remember mass and weight ARE different concepts!

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