Multiplying a 2digit number by
10001
and
adding 21
The product of a 2digit number and 10,001 + 21 is:
 the two digits,
 two zeros,
 the two digits,
 add 21.
Example: 29 × 10001 + 21
 29 × 10001 = 290029
 290029 + 21 = 290050
Multiplying a 3digit number by
10001
and
adding 201
The product of a 3digit number and 10,001 + 201 is:
 the three digits,
 one zero,
 the three digits,
 add 201.
Example: 436 × 10001 + 201
 437 × 10001 = 4370437
 4370437 + 201 = 4370638
Multiplying a 4digit number by
10001
and
adding 2001
The product of a 4digit number and 10,001 + 2001 is:
 the four digits, repeated once
 add 2001.
Example: 6741 × 10001 + 2001
 6741 × 10001 = 67416741
 67416741 + 2001 = 67418742
Create variations by changing the number to be added or subtracted  just make
the number an easy one to handle mentally.
