Answer:
protons+neutrons= mass number, so if the mass number is 13 and protons are 6 its 13-6=7 neutrons
Step-by-step explanation:
mass number: the sum of the number of protons and
neutrons in an atom
this is key as it explains that protons+neutrons= mass number, so if the mass number is 13 and protons are 6 its 13-6=7 neutrons