nailsNot what you think. We are talking about a measure of the size of nails that uses the symbol, d.

Why is it called “penny”? In England before the year 1488, the nail size was the price in pennies of 100 nails of that size. For example, 100 nails cost “16 pennies” and were named 16d. In addition, nails were originally manufactured square, not round.

Goal: Write the equation that converts “penny” to nail length.


  1. As a group, you are to measure each nail length to the nearest quarter inch. Complete the table.
Penny #


  1. Graph on your own graph paper and draw the line of best fit.



  1. Calculate the slope and the y-intercept



  1. What is the real life meaning of slope?



  1. Write the equation of the line of best fit.



  1. Predict the length of a 20d nail using your equation.




Teacher notes: Each group of students will need a bag of nails that includes one each of 4d, 6d, 8d, 10d, 12d, 16d. You can have more or less sizes depending on what you have laying around. Each nail needs to be labeled with the penny size using masking tape unless the students already know the sizes. Students will need graph paper and a ruler. You may want some 20d nails to test their prediction for #6.


Math Standard: G.MG.2 Line of best fit. Writing an equation of a line.

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