A number sentence is any addition, subtraction, multiplication or division problem  complete with the factors that are being computed  as well as the answer to the problem. The different parts of a number sentence are distinguished by the particular names assigned to them. In addition sentences, the factors being added are called addends, and the answer is called the sum.
The Addends

The factors of an addition sentence that are being added together are called the addends, and will be separated by a plus (+) sign. There can be any number of addends in an addition sentence. In the generic addition sentence a + b + c = d, "a", "b" and "c" are the addends.
The Sum

The sum is the solution to an addition sentence, or the answer computed when all addends are added. For a + b + c = d, "d" is the sum. There can only be one sum to any particular addition sentence, and it always follows the equals sign.
Finding a Missing Addend

In addition sentences with a missing addend, the addend can be solved for algebraically by isolating the variable and using the reverse operation, subtraction. For example, in the number sentence, 12 + x = 29, the missing addend "x" can be solved for by the subtraction sentence 29  12 = x, which equals 17.
Multiple Addends

In an addition sentence with multiple addends, add the first two addends, then add the sum of those two to the third addend. Do the same for any additional addends. For example, for the problem 3 + 9 + 11 + 7, adding 3 + 9 gives 12. Add 12 to the following addend, 11, to get 23. Finally, add 23 to 7 to get a final sum of 30.
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