Question 263006
That is the wrong formula for surface area because we will never be covering the bottom of the cake and we also need to subtract the area of the layer above.
pi*r^2
3.14*6^2
pi*r^2
3.14*12^2
sa=2*pi*r*h+pi*r^2

sa=2*3.14*20*5+3.14*20^2
sa=2*3.14*12*4+3.14*12^2
sa=2*3.14*6*3+3.14*6^2
add those three and subtract 
both 
3.14*6^2
3.14*12^2

so we just add
sa=2*3.14*20*5+3.14*20^2
sa=2*3.14*12*4
sa=2*3.14*6*3