Tuesday, May 7, 2013

Mental math with addition.

I have been very lucky to understand how to do arithmetic mentally, but maybe some people haven't been as lucky.  Here is a simple example for a young child to understand an addition problem.  By the way, mental math really only involves thinking about the nearest 5 or nearest 10.

Say we have the problem 7+8=
When we look at a group of objects, our eyes tend to group things into small groups, normally 2 and 3.  We also tend to sort things by color.  Looking at this group, mentally I would separate it as 3+2 blue is 5, 2 green, 3+2 pink is 5, and 3 gold.  It would look like this...
Knowing how to add 5s is really easy so we group those together--and addition is fun because we can easily move the numbers around (Commutative Property).  Our problem then becomes this.
And we see that it is just 5+5+5 which is 15.  Then if your child is ready you can show them they can do multiplication with this one...

Mental arithmetic can become easier if you practice and just think about how close the digits are to an easier number to work with.  For example 63+19: Separate into 60 and give the 3 to the 19 to be 22 and now the problem is 60+22.

I hope you enjoyed the addition mental math session!

How are you when it comes to doing arithmetic?  Can you do it mentally or do you need to write everything down?  Or do you pull out a calculator? 