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.
