The lessons here use the base ten block online manipulative:
http://nlvm.usu.edu/

Subtraction with regrouping is an algorithm that can be difficult for many students. When introducing subtraction problems to students, it is important to realize the level of difficulty of the problem. A problem such as 78 - 15 does not cause problems for most students because there is no need to regroup. Once regrouping begins, the problems tend to arise.

	Using the online manipulatives, the blue blocks represent the original amount, and the red blocks represent the amount to be subtracted. When a red block and blue block are placed together, the blocks disappear. It is clear to students that in this case, there are not enough ones in the top number to subtract 6. Therefore, they will need to regroup.
	The student moves one of the blue tens to the ones place, and the block separates into ten one blocks, leaving 14 one blocks on top. Now, the student can match up all of the blue ones with red ones. As the students perform the steps with the blocks, the program records the subtraction algorithm on the side.
	Once all of the regrouping and subtracting is completed, the program shows the correct answer in numbers as the student has shown the correct answer with the blocks.
Regrouping across zeroes poses many problems for students who do not understand the subtraction algorithm. The student sees that there are not enough ones in the top number, but there are also no tens to regroup.
The student recognizes that to get tens, a hundred must first be regrouped to the tens place. When the student moves the hundred block to the tens place, it splits into 10 ten blocks.
Now, one of the ten blocks can be regrouped as ones, leaving 9 ten blocks and 13 one blocks.
Now that all of the regrouping has been completed, the ones can be subtracted. 13 ones minus 5 ones leaves 8 ones.
The students then use the blocks to subtract the tens and hundreds, leaving a difference of 238.