Addition of numbers close to multiples of ten (e.g. 19, 29, 38, 59 etc.)
This technique is useful for Mental calculations
116 + 39 (Here we can write this 39 as 40-1)
= 116 + (40 - 1)
= 116 + 40 - 1
= 156 - 1 (Instead of adding 39 to 116, we just add 40 to 116 (because we can do this without using pen and paper) and later we subtract one from it)
= 155
Try this.. This is very useful tip while doing calculations.
Now lets try another example.
116 + 97
= 116 + (100 - 3)
= 116 + 100 - 3 (Here, instead of adding 97 to 116, we are just adding a 100 to 116 and then subtracting 3 from it :)
= 213
Addition of decimals
12.5 + 6.25
= (12 + 0.5) + (6 + 0.25)
= 12 + 6 + 0.5 + 0.25 (Here we just added the rounded numbers first and later we added the decimal numbers :)
= 18 + 0.5 + 0.25
= 18.75
