Question

Which of the following symbols results in a true number sentence when placed in the blank?

Answer 1

The term is used as means of asking students to write down equations using simple mathematical symbols (numerals, the four basic mathematical operators, equality symbol)[5]. Sometimes boxes or shapes are used to indicate unknown values. As such number sentences are used to introduce students to notions of structure and algebra prior to a more formal treatment of these concepts.

A number sentence without unknowns is equivalent to a logical proposition expressed using the notation of arithmetic.

[edit] Examples
A valid number sentence that is true: 3 + 7 = 10.

A valid number sentence that is false: 7 + 9 = 17.

A valid number sentence using a 'less than' symbol: 3 + 6 < 10.

An example from a lesson plan:

Some students will use a direct computational approach. They will carry out the addition 26 + 39 = 65, put 65 = 23 + □, and then find that □ = 42.[6] (wikipedia)

I hope this is helpful!