This picture shows one strategy to calculate 27 + 59 = 86.
<a href="//commons.wikimedia.org/w/index.php?title=User:LitFlower171&amp;action=edit&amp;redlink=1" class="new" title="User:LitFlower171 (page does not exist)">LitFlower171</a>
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<span class="int-own-work" lang="en">Own work</span>, <a href="//commons.wikimedia.org/wiki/File:Addition_Demonstration.pdf" title="File:Addition Demonstration.pdf">Addition Demonstration.pdf</a>