The picture has been reduced to 90% or $\simplify{{prop1}/100} \left(=\frac{\var{prop1}}{100}\right)$ of its original size.

So to find the size of the first copy we multiply the original size by $\simplify{{prop1}/100}$.

If we then reduced the new size by 20%, the final copy would be 80% of the size of the first copy, i.e. $\simplify{{prop2rel}/100}$ 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 $\simplify{{prop1}/100}$ by $\simplify{{prop2rel}/100}$ to get

$$\simplify{{prop1}/100} \times \simplify{{prop2rel}/100} = \simplify{{prop1*prop2rel}/10000}$$

Now to express this as a percentage we multiply by 100 and we obtain:

$$\simplify{{prop1*prop2rel}/10000} \times 100 = \var{final}\%.$$

So the final copy is 72% of the size of the original picture.