swerve wrote:A certain lighthouse is a tall thin cylinder of brick, with a light chamber at the top. The brick extends from the ground to the floor of the light chamber. The brick siding of lighthouse is in need of painting. How many square feet of brick does it have?
(1) The outer diameter of the brick portion of the lighthouse is 28 ft
(2) There are 274 steps from ground level to the floor of the light chamber, and each one is 8 inches high.
The OA is C.
Please, can anyone explain this DS question? I need help. Thanks.
Hi Swerve,
The question is ambiguously phrased, which won't happen on the official GMAT. Presumably, the question asks for the surface area of the lighthouse's outer wall. Since the question asks for a value, consider what is known from the question stem, and then consider what additional information is needed from the statements to answer the question.
The lighthouse is cylindrical, and the formula for the surface area of a right circular cylinder (excluding the top and bottom) is
height *
circumference.
Thus, in order to answer the question, the statements must provide information sufficient to determine the lighthouse's height and circumference.
According to Statement (1), the outer diameter of the lighthouse is 28 ft. The diameter of a cylinder can be used to find its circumference (circumference = π(diameter)), but Statement (1) provides no information about the cylinder's height, so Statement (1) is insufficient.
Write down BCE.
According to Statement (2), the height of the lighthouse is equal to 274 * 8 inches. However, Statement (2) provides no information about the cylinder's circumference, so Statement (2) is insufficient.
Eliminate choice B.
Finally, since Statement (1) is sufficient to determine the circumference of the lighthouse, and Statement (2) is sufficient to determine its height, both statements together are sufficient answer the question (provided you remember that 1 ft. = 12 inches.
Hope this helps.
Best,
