Each number can be represented using base 10 blocks.
Here are some numbers represented using base ten blocks.
What happens when you have a number like 49?
What would happen if I added another cube to 49?
Once you have 10 cubes you can trade the 10 cubes in for one long.
Now you would have this...
When you have 10 or more cubes,
you can trade
Notice that you have one more long and no cubes left (Remember, we traded the 10 cubes in for one long.)
If I add 2 more cubes to the number 28, I would have 10 cubes. I could then trade those 10 cubes for one long. Therefore you are regrouping. My answer would be 30, and I would have no cubes left over.
What if I added 3 to the number 28?
Would I have to regroup? Yes!
Now I can trade ten cubes in for one long, and I would still have one cube left over. My answer would look like this.
31 has three groups of ten (longs) and 1 one (cube)