That&#x27;s meant to be &quot;12 + 3 = 15&quot; (or maybe it&#x27;s an intentional error to make the thing look more confusing?).  The idea behind this particular subtraction algorithm is to reinforce two number-sense ideas: 1) a subtraction problem like &quot;32 − 12 = ?&quot; is equivalent to an addition problem &quot;12 + ? = 32&quot;, and 2) we can group additions in ways that are convenient to use numbers that are easy to add or to use convenient intermediate results that are multiples of 5 or 10 or 100.<p>EDIT: to fix my own math typo. :P