Adding a One Digit Number to a Two Digit Number with No New Ten Needed

Here is where this shows up in real life. Adding a few more items to a partly filled crate does not change how many full crates you have. This is worth learning because knowing when the tens stay still keeps two digit work simple.

Watch for it today, and when you spot it, tell someone what you noticed and how you knew. Ask yourself where else the same idea might be hiding, then go and look.

Get your eyes and your fingers ready. When the ones stay under ten, two digit addition is easy. Take 34 plus 5. Leave the three tens alone and add the ones: four plus five is nine.

The answer is 39. The tens never moved, because you never gathered enough ones to make a new bundle. Build it with rods and cubes to see it: three rods stay put while the cubes grow from four to nine.

Try this check before you start. Look at the ones digits and ask whether they will pass ten. In 34 plus 5, four and five make nine, so no new ten is needed and the tens can stay asleep. Checking first tells you what kind of problem you are facing. You will use this idea again and again, so build it strong today.