The picture has been reduced to 90% or 910(=90100) of its original size.
So to find the size of the first copy we multiply the original size by 910.
If we then reduced the new size by 20%, the final copy would be 80% of the size of the first copy, i.e. 45 of the size of the first copy.
To get the size of the final copy as a proportion of the size of the original copy, we multiply 910 by 45 to get
910×45=1825
Now to express this as a percentage we multiply by 100 and we obtain:
1825×100=72%.
So the final copy is 72% of the size of the original picture.