How much do membrane invaginations (folding) increase surface area?

There are two cells cell A and Cell B. Cell A is a rectangular prism and Cell B is the same thing but with invaginations in the membrane. Dimensions for cell A are: height = 3 um width = 6 um and depth = 2 um

i) calculate SA for cell A : I got 72 um^2
ii) calculate V for cell A : I got 36 um^3
iii) calculate SA/V for cell A : I got 2

Now they say that the volume of Cell B is 10 um^3 less than Cell A because of the folding and the SA/V is 48 , so what is cell B's SA

I did this:

SA/26 = 48
SA =1248 um^2

doesn't this seem ridiculous ?? Is this correct and it's just crazy or did I go wrong somewhere?