4. Using inverses

• Addition and subtraction are inverses (opposites). So subtraction can always be checked by adding:
  To check 82 - 37 = 45, you could add 45 and 37 and see if you get 82.
• Multiplication and division are also inverses. So division can always be checked by multiplication:
  To check 81 ÷ 3 = 27, you could multiply 27 by 3 and see if you get 81.