Explanation:
basically first you have to understand these definitions
*mass number ( e.g. where it says oxygen 16 or 17 - it's basically that number at the top)
mass number is the 'number of protons + number of neutrons'
now this will help you do the first question
if you do 16 -8 = 8 ( that's how she got 8 neutrons)
another thing you need to know is that all the time the number of protons = the number of electrons ( they are the same)
this will help you answer table 2
for fluorine it says there are 9 electrons, you automatically have to think 9 electrons means 9 protons too, ( I think she has got that wrong, it's supposed to be 9 protons not 19)
if you have 9 protons and the given 10 neutrons and add them together you will get the mass number which is supposed to be 19
then if you try out the same rules for argon you will see they work - the number of electrons and protons are same and when you find out the protons, you can just take that away from the mass number ( which is number of protons + neutrons) to get just the number of neutrons.
Hope this helps you :)
