Using the Montessori Decimal Board

Our 5th graders are working on adding and subtracting decimals using the Montessori Decimal Board. 

This board is made up of 13 columns with an extra dark line at the point where a decimal point would be, dividing whole numbers from the fractional numerical units of tenths, hundredths and so on. Throughout various Montessori math works, green signifies units, blue is tens and red signifies hundreds.  In this work, the fractional columns to the right are the pastel version of the colors used for the whole numbers.

If students were to add (1 and 7/10) + (2 and 4/10), they would select 3 (1+2) green “unit” beads to be placed in in that first unit column to the left of the decimal point. They would then take 11 (7+4) pastel green beads to be put in the first column to the right of the decimal – the tenth’s column. Since there are 10 or more beads in that spot, they trade those 10 tenths in for a green unit, adding an additional green bead to the unit column, leaving just one bead left behind in the tenths column. The result is 4 beads in the units and 1 bead in the tenths with the result being =  four and one tenth. 


     1     7/10

+  2    4/10

     4     1/10



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