If the numerator of a fraction is a prime number can the fraction be simplified and why?

Yes!!! it can . provided the denominator is a multiple of the numerator. e.g. 3/9

'3' is a Prime number, but '9' is a multiple of '3', and so can be cancelled down to 3/9 = 1/3 .

However. is both the numerator and the denominator are prime numbers they cannot be canclled down. e.g. 3/7 or 3/11 or 7/11 et.seq.,