FOUR PHASES IN SOLVING A PROBLEM,
From the book: How To Solve It, George Polya, 1945

(1)    Understand the problem

• What are the data?
• What is the unknown?
• Is there enough information to enable you to find a solution?
• Can you restate the problem in your own words? 

(2)    Devise a plan.

• Do you know a related problem?
• Look for a pattern
• Use a model
• Work backwards
• Use a formula
• Use direct reasoning

(3)    Try It

• Carrying out your plan of the solution.
• Can you prove that it is correct?

(4)    Look Back

• Examine the solution obtained.
• Does your answer make sense?
• Is it reasonable?
• Can you check the result?
• If you missed something check your plan and try the problem again.