Dr. Benjamin Arthur ,of the book The Secrets of Mental Math, encourages kids and adults alike to try adding left to right instead of right to left. The reason is simple. We are taught to read left to right, we write left to right, and are therefore more adept at thinking left to right. Only in math do we reverse the routine.
Here is how left to right addition works. Take an addition problem, such as 34 + 45. Add the tens columns first (34 + 40 = 74). Now add the ones column (74 + 5 = 79).
Try these examples in your head. I will pitch in some three digit numbers as well.
______23 62 134
_____+42 +26 + 63
This is all very good, but what if I throw in much bigger numbers, such as…
Not only are the numbers bigger, but they include carrying-over. Here’s a trick.
Round-up the bottom number to 300. Now add 300 to 256. Then subtract the difference between 300 and 289, which is 11.
Hence, 256 + 289 = 256 + 300 – 11.
Then 256 + 300 = 556
556 -11 = 556 – 10 -1
556-10 = 546 and finally 546 -1 =545 Your answer is 545.
Try these: 645 + 276 and 327 + 499
What we learned:
* Adding left to right can be faster.
* With big numbers it is often quicker to round up the bottom number and subtract the difference from the sum.
On Friday I will share how you can build on left to right math with subtraction!