SPEED MULTIPLYING WITH 21, 31, 41........

To multiply by 21:   Double the number, then multiply by 10, and add the original number.

for example,  to multiply 42 by 21,

      Double 42 yields 84,  when we multiply it with 10 we get 840, and then add the original number 42 we get 882.

To multiply with 31:  Triple the number, then multiply with 10, and add the original number.

For example, to multiply 17 by 31,

       triple of 17 is 51, multiplying 41 by 10 we get 510, and we get 527 when we add 17- the original number with 510.  

To multiply by 41: Quadruple the number, then multiply by 10, and add the original number