Numerator of a fraction after simplification : numer
getNum

numer or getNum takes as argument a fraction and returns the numerator of this fraction after simplification (for rational fractions, see 1.28.2).
Input :

numer(42/12)

Or :

getNum(42/12)

Output :

7

To avoid simplifications, the argument must be quoted (for rational fractions see 1.28.1).
Input :

numer('42/12')

Or :

getNum('42/12')

Output :

42