**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.