Multiply by 11 exercise

Rules to multiply by 11

Case 1: When sum of the 2 digits is less than 10

  • The left digit of the result is the left digit of the original number
  • The middle digit is the sum of the 2 digits of the original number
  • The right digit is the right digit of the original number
For example if we were to multiply 21×11, 
  1. The left digit of the result is the left digit of the original number that is 2
  2. The middle digit is the sum of the two digits of the original number that is 2+1=3
  3. The right digit is the right digit of the original number that is 1
  4. so the result of 21×11 = 231.

Case 2: When sum of the 2 digits is 10 or more

  • The left digit of the result is 1 more than the digit of the original number
  • The middle digit is the right digit of the sum of the 2 digits of the original number
  • The right digit is the right digit of the original number
For example if we were to multiply 86×11, 
  1. The left digit of the result is 1 more than the left digit of the original number. The left digit of the original number is 8 and the sum of the two digits of the original number is more than 10, so the left digit of the result is 1 more than 8 that is 9
  2. The middle digit of the result is the right digit of the sum of the two digits of the original number. The sum of the two digits of the original number 86 is 14. so the middle number of the result is the right digit of the sum of 8+6=14, which is 4.
  3. The right digit is the right digit of the original number that is 6
  4. so the result of 21×11 = 946.


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