Math Club 9/25 and 10/2 Summary

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

Trick: Multiply a 2-Digit # by 11 Process: Examples: 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 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 0s If you have a remainder after dividing, the last two digits of the answer are the remainder x 25 25 x 16 = 400 25 x 36 = 900 25 x 22 = 550 25 x 49 = 1225

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

Trick: 2-Digit Squares Ending in 5 Process: Examples: 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) 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 Last = 2 x 1 = 2 Inside + Outside = (2 x 3) + (5 x 1) = 11 (carry the 1 for the next step) First = (5 x 3) + 1 = 16 1612 35 x 74 Last = 5 x 4 = 20 (carry the one for the next step) Inside + Outside = (5 x 7) + (3 x 4) + 2 = 49 (carry the 4 for the next step) First = (3 x 7) + 4 = 25 2590

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

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

Trick: Ones place add up to 10, Tens place are equivalent (bump-up) Process Examples Your answer has 4 digits The last two digits are the ones places multiplied The first two digits are the tens place multiplied by one plus the tens place 54 x 56 = (5x6) (4x6) = 3024 81 x 89 = 7209 33 x 37 = 1221 48 x 42 = 2016

Trick: Multiplying 2 numbers in the 90s (ghost numbers) Process: Example: 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 Multiply the ghost numbers – those are the last two digits of the answer Subtract either ghost number from the digit – they will give you the same answer. Those are the first two digits of the answer 93 x 95 = 8835 92 x 97 = 8924 96 x 99 = 9504 5 7 3 8 1 4