This technique can be used when one of the numbers being added is very close to an even ten, hundred, thousands, etc. boundary. For example, 398 is just 2 less than 400.
This fundamental technique can be used in special cases to add numbers large and small. Once mastered it will help speed up all other trick calculations.
Step 1
Identify that one of the numbers is close to an even number and calculate the difference.
398 is just 2 less than 400.
Step 2
Use the rounded number in the original calculation to make it much easier.
586 + 400 = 986
Step 3
Subtract the result found in step 1 from the result in step 2.
986 - 2 = 984
The technique can be modified and used for a number that is slightly larger than a rounded number. For example, 402 is just 2 larger than 400. In this case the difference would be added back in step 3.
This technique, while simple, can be much faster than technique #1 or long addition for the cases in which it applies.