Prove that, if first and last numbers out of 3 consecutive numbers are prime, the middle number is divisible by 6.
say $x-1, x, $and$ x+1$ are the numbers, then,
if $x-1$ and $x+1$ are prime numbers,
prove $x$ is divisible by 6.
an exception: (3, 4, 5): It is true for all but this case.