Prime numbers are used in calculating the Highest Common Factor
Basically, you have to divide each number BY IT'S SMALLEST PRIME until you end up with a prime itself...
now if 1 were a prime, you'd be there all day, saying
356/1 = 356
356/1 = 356
356/1 = 356
356/1 = 356
356/1 = 356
356/1 = 356
356/1 = 356
356/1 = 356