1. Math Club
9/25 and 10/2 Summary

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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 ( ) =
101 x 924 = 93324
101 x 407 = 41107
101 x 315 = 31815

9. 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

10. 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

11. 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 = x 97 = x 99 = 9504