At first glance #57# would seem to be a prime number ....

after all, #7," "17," "37," "47," "67," "97# are all prime.

Why not #57?#

If you add the digits of #57# you get #5+7 = 12#

#12# is a multiple of #3#

This means that #57# is also a multiple of #3#

The nearest multiples of #3# to #57# are:

#60# - which is #3 xx20# and #54# - which is #3xx18#

#57# is #3# away from both of these and is actually #3xx 19#

There are #25# prime numbers from 1 to 100, but there are #5# numbers which warrant extra attention, because they look as though they might be prime but are actually not. Learn them!

These are #" "color(red)(1," "51," "57," "87," "91)#

#1# has only one factor, not two factors as prime numbers have.

#51, 57 and 87# are all multiples of #3#. (add their digits)

#91 = 7 xx13#

As it is a product of two primes and above the normal times tables, we will not often have come across #91# in our maths.