At first glance 5757 would seem to be a prime number ....
after all, 7," "17," "37," "47," "67," "977, 17, 37, 47, 67, 97 are all prime.
Why not 57?57?
If you add the digits of 5757 you get 5+7 = 125+7=12
1212 is a multiple of 33
This means that 5757 is also a multiple of 33
The nearest multiples of 33 to 5757 are:
6060 - which is 3 xx203×20 and 5454 - which is 3xx183×18
5757 is 33 away from both of these and is actually 3xx 193×19
There are 2525 prime numbers from 1 to 100, but there are 55 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, 51, 57, 87, 91
11 has only one factor, not two factors as prime numbers have.
51, 57 and 8751,57and87 are all multiples of 33. (add their digits)
91 = 7 xx1391=7×13
As it is a product of two primes and above the normal times tables, we will not often have come across 9191 in our maths.
