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Students learn to add numbers with two or more digits, such as 139 + 25. The first step is to line up the numbers vertically so that the units digits are in the same column. Next, add the units digits, the tens digits, and the hundreds digits. Adding the units digits, 9 + 5 = 14, which is the same as 1 ten and 4 ones, so put a 4 in the units column and carry a 1 to the tens column. Now, adding the tens digits, 1 + 3 + 2 = 6, and adding the hundreds digits, 1 + 0 = 1. So 139 + 25 = 164. Note that the answer to an addition problem is called the sum, so the sum of 139 + 25 is 164.