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
4 = ? + 4
Next, it would read
4 - 4 = ? + 4 - 4
and the answer to the problem is zero.