Captivating Sum

Captivating Sum

That it is easier to add round numbers like 40 or 50 than numbers ending in 7, 8, or 9. If you round these awkward numbers up by adding 3, 2 or 1 the calculation is not much longer, and is easier.
Instead of
49+52 = 101
think of it as
50 (that is 49+1) + 52 = 102-1 = 101

Other example
If you are adding
215
426
513
112
328
----

First add the figure in the hundreds column and hold the total, 1500, in your head. Now add the total of the tens column, 70, to it. To this total 1570 add the sum of the units column, 24, to arrive at the final total of 1594