Q24 on MGMAT Q-Bank, called "Surface Comparison" is as follows:

Cylinder A, which has a radius of x and a height of y, has a greater surface area than does Cylinder B, which has a radius of y and a height of x. How much greater is the surface area of Cylinder A than that of Cylinder of B?

This problem assumes that the 2 cylinders being compared are solid cylinders and not hollow cylinders (in which case we would not add the surface area for the top and bottom). Is this a standard assumption we could make for all GMAT cylinder problems?

Thanks for any comments.

You are right! Unless it is explicitly mentioned we should assume that the cylinder has 2 circular faces. Also on your point on hollow cylinders - A cylinder can be hollow and still have 2 circular faces Ex: Any empty coke can ;-) It all depends on what object is visualized as a cylinder. Some examples of the common visualizations for cylinder I have seen is:

1) An empty cylindrical drum with a face open. This is a cylinder with one circular base and hence we should consider only one circle in calculating the area.

2) A cylindrical pipe which has got a certain thickness. This can be considered as a hollow cylinder which will have concentric circular base (You should calculate the circular base area by subtracting the area of inner circle from outer circle).

Hope it helps!

So in this case SA = pi*x^2*y + 2 * pi* x^2
and SB = pi * y^2*x + 2 *pi * y^2

and then Sa - Sb - is that correct?

I did not think of it as a solid cyclinder till I read ur explaination!!!

thanks so much!

to address part of the original poster's question - yes, you assume on the test that a given shape is solid (all relevant surfaces) unless they tell you otherwise. A cylinder, a pyramid, a sphere - whatever.