easy peasy math (i do math weird in my hea so hopefully i convey this in a way you understand as well)
if 15 is 20% of the total emails he has 80% left right.
20% is 1/5 so you need to find the other 4/5
to do this you take the 1/5 ammount (15) and times that by 4
so thats 15 times 4
which is 60
he has 60 emails left
if the question is how many he has left then the answer is 60
if the answer is how many total emails then you add the 15 hes answered already to the 60 he needs to answer still and its 75