Missing number problems
In a nutshell
Expressions involving an unknown number are needed to solve missing number problems. Unknown numbers are usually represented by letters.
Example 1
$x + 10 = 12$
This is the same as asking: "what is added to $10$ to get $12$?"
The answer is $x=2$
Note: $x$ is the letter commonly used for an unknown.
Method for solving
Procedure
1.
 Write out an equation 
2.  Work out the value of the unknown

Example 2
Anna has $£20$. She shares $£5$ with her brother and uses $£12$ to buy a new dress. how much money does she have left? Write an expression: $5 + 12 + x = 20$
Solve for $x$: $x= 3$
Anna has $\underline{£3}$ remaining. 