Adding a Multiple of Ten to Any Two Digit Number

Here is where this shows up in real life. Adding two dimes to a pocket of change increases the dimes and never touches the pennies. This is worth learning because knowing what does not change is as useful as knowing what does.

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.

Here is something worth knowing today. Adding tens to a number with ones in it leaves the ones alone. Take 43 plus 20. Add two tens to the four tens, giving six tens, and the three ones stay exactly as they were. The answer is 63.

Look at the ones digit before and after: unchanged. Whenever you add whole tens, only the tens digit moves. That is a rule you can trust every time.

Watch out for touching the ones. In 43 plus 20, the three must still be a three when you finish. If your answer has a different ones digit, something moved that should have stayed still, and it is worth finding out what. Practice this a few times and it will start to feel easy.