Ctrl+k

Use a symbol for an unknown

A symbol such as a box, question mark, or letter can represent an unknown whole number in an equation, whose value is determined by the relationship that makes the equation true, whether it appears before, between, or after known numbers (for example, □ + 7 = 12 or 15 − □ = 9). This understanding applies to addition and subtraction with whole numbers within 100 and supports reasoning about equality and inverse operations, not generalized variables, negative numbers, or formal algebraic manipulation.

Detailed Explanation: Use a symbol for an unknown

A symbol can stand for a number we do not know yet. We find its value by making the equation true.

Example:

15−□=915-\square=9
  1. Read the equation: “15 minus a number equals 9.”
  2. Think: What number can we take away from 15 to leave 9?
  3. Count up from 9 to 15:
9+6=159+6=15

So, the missing number is 66.

Check the answer:

15−6=915-6=9

The equation is true, so 6\boxed{6} is the value of □\square.

Learn by doing: Use a symbol for an unknown

Click a topic below to practice the foundational skills you'll need, learn the steps, or master this skill

    Practice with unlimited practice problems

    Linear Equation - Solve for Box, Two Terms, Add and Subtract

      ?