Math Club 925 and 102 Summary Trick Multiply

Math Club 9/25 and 10/2 Summary

Trick: Multiply a 2 -Digit # by 101 (The easiest trick in the book) Process: The two digit number repeats itself! Examples: 101 x 73 = 7373 101 x 26 = 2626 101 x 99 = 9999

Trick: Multiply a 2 -Digit # by 11 Process: Add the two digits together Put that digit in between the number multiplying by 11 **If the sum of the two digits is > 9, carry the tens place and add it to the hundreds place Examples: 11 x 17 = 187 11 x 36 = 396 11 x 64 = 704 11 x 88 = 968

Trick: Multiply a 2 -Digit # by 25 (4 quarters in a dollar) Process: Examples: Divide the number by 4 and add two 0 s 25 x 16 = 400 If you have a remainder after dividing, the last two digits of the answer are the remainder x 25 x 36 = 900 25 x 22 = 550 25 x 49 = 1225

Trick: Multiply a 2 -Digit # by 50 Process: There are two groups of 50 in 100. Divide the number by two and add two 0 s Examples: 50 x 48 = 2400 50 x 24 = 1200 If the number is even, the answer will end in 00 If the number is odd, the answer will end in 50 x 33 = 1650 50 x 85 = 4250

Trick: 2 -Digit Squares Ending in 5 Process: The last two digits of the answer are always 25 (5 x 5) Multiply the tens digit times one more than the tens digit If the tens digit is 6, you would multiply 6 x (6+1) If the tens digit is 8, you would multiply 8 x (8+1) Examples: 352 = 35 x 35 = 1225 552 = 55 x 55 = 3025 752 = 75 x 75 = 5625 852 = 85 x 85 = 7225

Trick: Multiply a 2 -Digit # by a 2 -Digit # (LIOF – Last, Inside + Outside, First) Process and Examples: 52 x 31 35 x 74 Last = 2 x 1 = 2 Last = 5 x 4 = 20 (carry the one for the next step) Inside + Outside = (2 x 3) + (5 x 1) = 11 (carry the 1 for the next step) Inside + Outside = (5 x 7) + (3 x 4) + 2 = 49 (carry the 4 for the next step) First = (5 x 3) + 1 = 16 First = (3 x 7) + 4 = 25 1612 2590

Trick = 101 x a 3 -digit # Process: Examples: Your answer will have 5 digits 101 x 924 = 93324 101 x 542 = __ __ __ 101 x 407 = 41107 Copy the last two digits: __ __ __ 4 2 101 x 315 = 31815 Add the hundreds place to the 3 -digit number (542 + 5) = 5 4 7 4 2

Trick: 11 x a 3 -digit # Process: Your answer will have 4 digits 412 x 11 = 4532 263 x 11 = (2) (2+6) (6+3) (3) = 2893 671 x 11 = 7381 If the answer is greater than nine, carry the tens place over to the next digit 907 x 11 = 9977 572 x 11 = (5+1) (5+7) (7+2) (2) = 6292 855 x 11 = 9405

Trick: Ones place add up to 10, Tens place are equivalent (bump-up) Process Your answer has 4 digits The last two digits are the ones places multiplied Examples 81 x 89 = 7209 33 x 37 = 1221 The first two digits are the tens place multiplied by one plus the tens place 54 x 56 = (5 x 6) (4 x 6) = 3024 48 x 42 = 2016

Trick: Multiplying 2 numbers in the 90 s (ghost numbers) Process: Remember – you can’t show any work on the number sense test; you’ve got to keep all the ghost numbers in your head! *** The ghost numbers are how far away each # is from 100. Write each ghost # underneath the opposite digit 1) Multiply the ghost numbers – those are the last two digits of the answer 2) Subtract either ghost number from the digit – they will give you the same answer. Those are the first two digits of the answer Example: 93 x 95 = 8835 5 7 92 x 97 = 8924 3 8 96 x 99 = 9504 1 4