The number 3:


  • Take any number: 9
  • Double it: 9 * 2 = 18
  • Add 6: 18 + 6 = 24
  • Take half of it: 24 / 2 = 12
  • Take away the number your started with: 12 - 9 = 3


The number 1089:

  • Take any 3-digit number where, from left to right, it descends: 321
  • Inverse the digits: 123
  • Subtract the two: 321 - 123 = 198
  • Inverse this result: 891
  • Add these two numbers: 198 + 891 = 1089


The number 11880:

  • Take any 4-digit number where, from left to right, it descends: 4321
  • Inverse the digits: 1234
  • Subtract the two: 4321 - 1234 = 3087
  • Inverse this result: 7803
  • Add these two numbers: 3087 + 7803 = 11880