Let's look at the number 42,135. Each column in the number has a name.
The 5 is in the units column.
The 3 is in the tens column.
The I is in the hundreds column.
The 2 is in the thousands column.
The 4 is in the ten thousands column.
When we total the columns, we have
In addition, when you write
4 Tens + 7 Units
3 Tens + 2 Units
you are saying
7 Tens + 9 Units
You are adding units to units and tens to tens.
One big trouble spot in addition is carrying.
If you do not understand carrying, the following sample should help you.