# What Is the Way to Find a Missing Number in a Math Problem?

**It depends on the operation being performed within the math problem, but finding a missing number typically requires the student to perform the opposite operation on both sides of the equation.** Finding a missing number is also known as solving an equation and is part of solving for an unknown variable in basic algebra.

For example, if the equation reads as 2 + ? = 15 to solve, the student would need to subtract 2 from both sides. Therefore, it would read 2 – 2 + ? = 15 – 2. When both sides are solved, it reads ? = 13. Therefore, the unknown number is 13.

In another instance, if an equation reads as ? – 3 = 9 both sides would have to have the number three added. It would then read ? – 3 + 3 = 9 + 3. When nine and three are added, the answer is 12, and that would then read ? = 12.

Unknown numbers can also be solved with more operations. For example, 2 + 3 – 1 = ? + 4 can be solved by subtracting four from both sides after the other operations have been completed. Therefore, 5 – 1 = ? + 4 followed by 4 = ? + 4 Next, it would read 4 – 4 = ? + 4 – 4 and the answer to the problem is zero.