first you take 1/6th of the original 90 copies which is 15 and subtract that from the 90, you are left with 75 copies which is how many they had left at the end of yesterday. then you take 1/5th of the 75 copies that are left which is also 15 and you subtract that from 75 and you get 60. that's why at the end of today there are 60 copies left.
