Landmark numbers
10  100 1000
25   50    75



Familiar landing numbers like
10  100 1000
25   50    75
200,210,220,230,240,250 or
300,400,500,600,700,800,900,
They are easy to work with

http://chetsmath.blogspot.com/2009/01/using-landmark-numbers-on-open-number.html