Chiarvalle Montessori Math Club

Welcome to everybody in math club and anyone else who's interested in mental math!

February 16, 2012

Today we learned some more addition and subtraction tricks. After you've practiced the hundred compliments, go back to the addition and subtraction problems. Try to use compliments in addition problems with carrying or subtraction problems with borrowing.

• Three digit number plus three digit number
  (Example: 477 + 387 = 864, since 477 + 300 = 777 and 777 + 87 = 864)
• Two digit number minus two digit number
  (Example: 57 - 38 = 19, since 57 - 30 = 27 and 27 - 8 = 19)
• Three digit number minus three digit number
  (Example: 574 - 382 = 192, since 574-300=274 and 274-82=192)
• Hundred compliments
  (Example: Complement of 32 is 68, since 3+6=9 and 2+8=10.
  
Here's an example of addition using a compliment:
477 + 387 = 864, since 477 + 400 = 877 and 877 - 13 = 864 (13 is the compliment of 87, so 387+13=400)
Here's an example of subtraction using a compliment:
574 - 382 = 192, since 574 - 400 = 174, and 174 + 18 = 192 (18 is the compliment of 82, so 382+18=400)

February 9, 2012

Today we reviewed some of the tricks we learned last time and added some new ones:

• One digit number plus two digit number
  (Example: 47 + 6 = 53, since 7 + 6 > 10 and 7 + 3 = 13)
• Two digit number plus two digit number
  (Example: 47 + 38 = 85, since 47 + 30 = 77 and 77 + 8 = 85)

January 26, 2012

Here are the mental math tricks we practiced today. Click on the links for some extra practice.

• Eleven times a two-digit number
  (Example: 11 * 62 = 682, since 6+2=8)
• Eleven times a three-digit number
  (Example: 11 * 341 = 3751, since 3+4=7 and 4+1=5)
• Square of a two-digit number ending in 5
  (Example: 65 squared is 4225, since 6*7=42 and 5*5=25)
• Twin tens, unit compliments
  (Example: 83 * 87 = 7221, since 8*9 = 72 and 3*7=21)
• One-digit number times a number between 10 and 20
  (Example: 6*13 = 78, since 6*10=60 and 6*3=18)
• Both numbers between 10 and 20
  (Example: 14*17=238, since 14+7=21, 21*10=210, 4*7=28, 210+28 = 238)