Saturday, June 16, 2012

Multiplying 3 Digit Numbers with Each Other in 30 Seconds



Part of my quest to conquer evil standardized testing has included the search for better multiplication methods. Recently I found the method below and thought I would work to spread the word.

Using the cross or vertical technique will save you half the time and scratch paper used by the standard technique most students are taught in grade school.

Cross or Vertical Multiplication

Use two, three digit numbers, let's multiply them.

Step 1: Vertically multiply the first two digits in each number.

Step 2: Cross multiply the first two digits in each number, then add their products, place the unit digit of the result in the solution and carry anything over. If carrying forward I usually write the tens digit first and then the unit digit, it’s just a personal preference)

Step 3: Multiply the three digits in each number with each other, then add the products. Note that each digit only has to be multiplied once and that we cross multiply when we can and multiply vertically when we can’t.

Step 4: Cross multiply the last two digits in each number and add their products, again remember to carry forward when appropriate.

Step 5: Vertically multiply the last digit in each number, writing the unit digit to the solution and carrying forward.

Step 6: Add down from right to left, carrying over if appropriate.

That's it! You're Done!