Can someone help me with when to use "Nummer" and when to use "Zahl" please?
Zahl = any number
Nummer = a number that forms part of a set of numbers e.g. the Hausnummer (house number), Telefonnummer (telephone number)
Ziffer = digit i.e. 0,1,2,3,4,5,6,7,8,9
In this case, both are correct.