Mentally Add 2 Digit Numbers

Adding two one digit numbers together is something, I assume, most of us can do easily in our head.  But how many of us can do two digit addition problems in our head?  For some it’s difficult, for some they may struggle, and for some it may be a breeze.

 

Hopefully this blog will help you to learn how to add two digits numbers together easily and mentally, possibly even faster than you could type the problem into a calculator!

 

To start learning how to do this, we have to remember two things:

1.. Simplify the problem

2.  Add left to right

 

WAIT! add left to right?! We were always taught to add right to left! <<<— Did you just think this?  That’s okay.  Adding right to left is okay for adding on paper but we read left to right and as you will soon see, that by simplifying the problem and reading left to right, the answer comes to you practically instantly!

 

The easiest two digit addition problems are those that do not require you to carry any numbers, when the first digits sum to 9 or below and the last digits sum to 9 or below.  Like the following:

adding-2-digits

To solve 47 + 32, first add 30 then add 2.  After adding 30, you have the simpler problem of 77 + 2, which equals, 79.  We can illustrate this as follows:

adding-2-digits-2

The above diagram is simply a way of representing the mental processes involved in arriving at an answer using this method.

 

Now let’s try a problem that requires you to carry a number…

adding-2-digits-3

Adding from left to right, you can simplify the problem by adding 67 + 20, then 87 + 8 = 95.

adding-2-digits-4

Now try one on your own, mentally calculating from left to right, and then check below to see how it was done.

adding-2-digits-5

How was that?! You added 84 + 50 = 134 and added 134 + 7 = 141, right?!

 

If carrying numbers trips you up a bit, don’t worry about it.  This is probably the first time you have ever made a systematic attempt at mental calculation like this, and if you’re like most people, it will take you time to get used to it.  With practice, however, you will begin to see and hear these numbers in your mind, and carrying numbers when you add will come automatically.

 

Try these sets of exercises for added practice.

adding-2-digits-6

 

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-Trevor.