A number sentence is a mathematical expression using numbers and operators like addition, subtraction, multiplication, or division. Number sentences are equal if they have the same answer, and equal number sentences can be formed using patterns of numbers that add or subtract to the same total. To solve a number sentence for a missing value, use the inverse operation - for example, if asked to find the missing value n in the sentence "2 + n = 5", subtract 2 from both sides to find that n = 3.