We regroup (borrow) numbers when we solve subtraction problems that cannot be solved as they are. For example, when the top digit in the ones place is less than the bottom digit in the ones place, you must regroup. But what if the digit in the tens place, or even in the hundreds place, is a zero? (You will sometimes see the terms "ones column", "tens column", and "hundreds column" used.)
Let's look at this problem.
The top digit in the ones place, 4, is less than the bottom digit, 5. We need to regroup using the tens place. But the tens place has a zero. Can we regroup with a zero?
We can't. We have to go to the first digit that is not a zero. In this case, we have to go to the digit in the hundreds place, which is 2. So we're regrouping with 20.
Let's cross out 20 and rewrite it as 19 plus 1 (19 + 1).
Now we can solve it. Adding the 1 ten to 4 equals 14: 10 plus 4 equals 14 (10+ 4 = 14). 14 minus 5 equals 9 (14 - 5 = 9).
Because there's nothing left to subtract, let's bring down 19, and we can finally get our answer.
Our difference (answer) equals 199.
Now let's try another problem, this time using a four-digit number with zeros.
The top digit in the ones place is less than the bottom ones digit, so we have to regroup from the next place value. But the next digits are zeros! So we regroup from the first digit that is not a zero, which is 5, in the thousands place (thousands column). We are going to rewrite 500.
We rewrote 500 as 499 plus 1 (499 + 1). Now we can subtract. The ones place is now 10 plus 3 equals 13 (10 + 3 = 13), and 13 minus 8 equals 5 (13 - 8 = 5).
The difference equals 4,995.
