Question 171060
Wish I could draw a sketch of this block as it would require less explanations.
Only the outside blocks will be painted. 
I've assumed the base is 9 by 12 with the height of 5.
The 2 ends are 2*5*9=90 blocks.
The 2 sides are 2*10*9=180 [the 10 allows for the ends being removed]
The top & bottom are 2*10*3=60 [the 3 allows for the sides being removed]
The sum of the outside blocks are:
90+180+60=330 blocks will be painted.