Memory Magic

The very simplest math trick is to multiply a number by one.  Any number times one is, of course, the same number. Every body knows that, but there are corollaries and implications of this trick that are often missed. 

 We often see something like 21 x 32 as a problem of multiplying by 21.  Actually in a simpler form it is a process of multiplying by 2 and then by 1.  Consider that 2 x 32 is equal to 64. We add a zero because it was actually 20 that we multiplied by.  So 20 x 32 = 640.  Now we just have to add 1 x 32 or 32.   640 + 32 =672.  This is much easier than trying to multiply by 21 and with practice can be done very quickly.   Conversely 19 x 32 is really 20 x 32 or 640 minus 1 x 32.  So 19 x 32 = 608.  When the concept is fully grasped it takes just a second to do the calculation.