1. A clever calculation method of multiplying a number by two digits by 1 1: the head and tail are the same, and the left and right are added and placed in the middle (if the ten digits are full, one digit should be advanced to 1).

For example: 24×11= 264 45×11= 495 56×11= 616.

Second, the ingenious calculation method of multiplying two digits by 99: add 1. Here, "go 1" means the original number minus 1, and find the business trip as the front of the number; "add" means writing the complement of the original number after the difference (100 minus the difference in this book).

For example: 58× 99 = 5742 57 = 58-142 =100-58.

Third, the clever calculation method of the head and tail and ten (the two digits are the same, and the digits add up to 10): the ten digits are multiplied by 1, and the digits are multiplied by the back (when the product is single digits, the front should be added with 0).

For example: 48× 42 = 201620 = (4+1)× 416 = 8× 2.

8 1×89=7209 72=(8+ 1)×8 09= 1×9

4. A number multiplied by 15 equals to the sum of 10 multiplied by this number plus half of it.

For example: 84×15 = (84+42 )×10 =1260.