You know that if you have three cookies and a friend has two cookies, you have five all together. This is simple addition. Sometimes, we can save ourselves a little bit of time by doing addition in our heads. When you're dealing with lesser numbers (smaller numbers), you can solve some problems even if you don't have a pencil and paper handy.
Here is a mental addition problem: 74 + 5. We have a two-digit number and a one-digit number. What is the ones digit for each number? In 74, the ones digit is 4. In 5, it's 5. So we just add 4 + 5 to equal 9. Do we have a tens digit in either of these numbers? It's 7. So we stick the tens digit, 7, in front of the ones digit, 9. Our sum (answer) equals 79.
Here's a problem that's a little trickier: 56 + 4. Again, let's look at the ones digit first. The ones digits are 6 and 4, which add up to equal 10. This means that the sum will have a 0 in the ones place (ones column), but we also have to regroup, or carry, a 1. What is the tens digit in these numbers? It is 5. So we add 5 to the 1 that we carried, and that equals 6. We put the tens digit, 6, in front of the ones digit, 0. Our sum equals 60.
