Len Greenberg, Song Writer

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Trick 1

What's 734 divided by 5?

Double 734: 1,468

Take away one decimal place: ANSWER:

Subtract 56: ANSWER: 504

Place "25" to the right: ANSWER: 5,625

How to multiply two numbers whose sum ends in "0:

In the preceding trick, we learned how to square a number that ends in "5". Situations like that don't arise every day but when they do, it's nice to be able to handle them.

Here's another situation that pops up every once in a while. Suppose you wanted to multiply two numbers whose sum ends in "0" -- numbers, for example, like 66 and 74, one ending in "6", the other in "4". Wouldn't it be nice to have a short-cut way of handling the calculation?

If the numbers you want to multiply are only two digits long and not very far apart -- less than 25 or so -- it so happens there

Step 1

Take half of the difference between the numbers you want to multiply and add it to the smaller of the two.

Some of you may be wondering why it’s important that the numbers you wish to multiply have a sum that ends in "0"? Here's why: if the sum of two numbers ends in "0", the number halfway between them must end in either a "0" or a "5". The fact that it ends in a "0" or a "5" makes it easy to square, even though it's two digits long. Squaring a two-digit number ending in "0" can be easily handled in one's head and we just learned in Trick No. 3 how to square numbers ending

in "5".

Experienced Math Tutor

I am available for math tutoring, from the elementary grades through college level. Please contact me at lennberg@verizon.net for more information.

Younger students enjoy learning with me because I make math fun! Advanced students benefit from my practical approach to learning complex theory and application.

I have taught math at Northern Virginia Community College for the past twelve years. I have a Bachelors Degree in Mathematics from the City College of New York and a Masters Degree in Mathematical Statistics from Columbia University. For many years I was a statistical consultant for both the government and private industry. I enjoy teaching much more than I did consulting!