First, calculate the area of the whole platform before the rectangular portion is removed:
Area = length * width
Area = 16 in. * 10 in.
Area = 160 in^2
Next, calculate the area of the removed rectangle:
Area = length * width
Area = 12 in. * 4 in.
Area = 48 in^2
Subtract the area of the removed rectangle from the total area to find the surface area of the platform:
Remaining area = total area - removed area
Remaining area = 160 in^2 - 48 in^2
Remaining area = 112 in^2
Since the platform is 3 inches thick, you will need to multiply this remaining area by 3:
Surface area = Remaining area * thickness
Surface area = 112 in^2 * 3
Surface area = 336 in^2
Therefore, the surface area of the platform if it is 3 in. thick is 336 in^2.