When adding and subtracting double-digit numbers, you sometimes end up with a small remainder. For example, when adding 17 and 24, you would generally start by adding the seven in the 17 and the four in 24, which gives you an 11. Instead of writing 11 in the answer space, you would write one and put the second one on top of the next digit on the left (be it the one in 17 or the two in 25). Then you would add the three numbers on the left side of the equation. A third-grader should have the knowledge to complete an equation with a remainder.
A third-grader will start learning about more complex aspects of subtraction, including deficits. While it is still too early for her to learn about negative numbers, some equations may include deficits. A deficit occurs when you have two numbers -- for example, 23 minus 16 -- stacked on top of each other. While 16 is a smaller number than 23, protocol dictates that you subtract the six from the three first. Instead of writing a negative number as the answer, she needs to borrow one value from the two in 23. Have her cross out the two and replace it with a one. Then she will add a one to the three, making its value 13. She can then subtract the six from the 13 and still get the correct answer for the equation.
In the third grade, children will learn how to subtract and add large numbers. The children are not ready for using numbers up in the thousands, but should be able to handle adding and subtracting in the hundreds range. Present the equation in a vertical fashion to make it easier to work with. This means the numbers in the equation should stack on top of each other and the answer should go below the original numbers.
A child in the third grade should be able to perform slightly more complex mathematical equations that include three different values. For example, he should be able to add seven, 15 and two. Just stack the three numbers on top of each other with the single digit numbers lining up with the second digit of a two-digit number. For example, when adding six to 26, the two sixes should stack on top of each other.