A general approach is to let the variable be whatever the question asks you to find......
Find the number. Let the number be #x#
How many toys does Pete have? Let the number of toys be #x#
What is the price of the shirt? Let the price be #x#
If you are asked to find two numbers,
or the length and breadth of a rectangle
or the ages of a mother and her son
or the price of pears and peaches, then.....
Let the variable be the smaller number and then write an expression for the other value.
When each unknown has been defined with a variable, then write the equation.
e.g. The sum of two numbers is 15:
Let the smaller number be #x# and the larger be #15-x#
The sum of three consecutive numbers is #57#
Let the numbers be:#" "x," " x+1," " x+2#
Sometimes you will not be given a specific relationship between the numbers. In this case use two variables.
Let the numbers be #x and y#