My grandfather taught it to me this way, when I was probably about 5; draw a line down the middle.
To take the square of any number ending in ...5: (15, 25, etc)
x | 5
x+1 | 5
=============
x * (x+1) | 25
[ Sadly, the formatter ignores leading spaces, evidently. ]
To take the square of any number ending in ...5: (15, 25, etc)
x | 5
x+1 | 5
=============
x * (x+1) | 25
[ Sadly, the formatter ignores leading spaces, evidently. ]
