Thirty One [Part Two]

In the previous post, Thirty One [Part One], it was stated that 31 is a centered triangular number; this is because we can represents 31 in the following figurative way (count the dots over the sides of the triangles and notice the order that the dots are imposed):


One basic (and sometimes useful) feature of 31 is how its multiplication with a number could be converted to 4 simple additions as illustrated in the following examples.

Example 1: 50 * 31 =

+050
+500
+500
+500

= 1550

Example 2: 44 * 31 =

+044
+440
+440
+440

= 1364

Example 3: 754201 * 31 =

+0754201
+7542010
+7542010
+7542010

= 23380231

Note: 31, 331, 3331, 33331, 333331, 3333331, and 33333331 are prime numbers; however, 333333331 is not! As 333333331 / 17 = 19607843.