What is Number sentence? - Definition Facts and Examples For example: 10 + 5 = 15. Here we are using the = sign which indicates a balance of both sides. However, there could also be number sentences which say: 12 + 6 = 9 is not true, but 12 + 6 = 18 is true. Therefore, a number sentence does not necessarily have to be true. However, every number sentence gives us information, and based on the ...