the floor area of the structure is 10 x 12 = 120 sq feet
the two walls will be. 5 x 12 x 2 = 120 sq feet
the other two walls will be 5 x 10 x 2. = 100 sq feet
so the whole area of the structure will be 120 + 120 + 100 =. 340 sq feet
since we use cubical blocks of 1 feet we will need 340 blocks