Matematik

Multiplication Tips

Multiplying by five

  • Jenny Logwoodwrites: Here is an easy way to find an answer to a 5 times question.If you are multiplying 5 times an even number: halve the number you are multiplying by and place a zero after the number. Example:5 × 6, half of6 is 3, add a zero for an answerof 30. Another example:5 × 8, half of8 is 4, add a zero for an answerof 40.If you are multiplying 5 times an odd number: subtract one from the number you are multiplying, then halve that number and place a 5 after the resulting number. Example: 5 × 7: -1 from 7 is 6, half of 6 is 3, place a 5 at the end of the resulting number to produce the number 35. Another example:5 × 3:-1 from 3is 2, halfof 2is 1, placea 5 at the end of this number toproduce 15.

     

  • Doug Elliott adds: To square a number that ends in 5, multiply the tens digit by (itself+1), then append 25. For example: to calculate 25 × 25, first do 2 × 3 = 6, then append 25 to this result; the answer is 625. Other examples: 55 x 55;5 × 6 = 30, answer is 3025. You can also square three digit numbers this way, by starting with the the first two digits:995 x 995;99 × 100 = 9900, answer is 990025

 Multiplying two 2-digit numbers                (same 1st digit)

  1. Select two 2-digit numbers with the same first digit.
  2. Multiply their second digits (keep the carry).   _ _ _ X
  3. Multiply the sum of the second digits by the first digit, add the carry (keep the carry).   _ _ X _
  4. Multiply the first digits (add the carry).   X X _ _

   Example:

  1. If the first number is 42, choose 45as the second number (any 2-digit number with first digit 4).
  2. Multiply the last digits: 2 × 5 = 10 (keep carry)   _ _ _ 0
  3. Multiply the sum of the 2nd digits by the first: 5 + 2 = 7; 7 × 4 = 28; 28 + 1 = 29 (keep carry)   _ _ 9 _
  4. Multiply the first digits (add the carry) 4 × 4 = 16; 16 + 2 = 18                           1 8 _ _
  5. So 42 × 45 = 1890.

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