Practice GMAT Data Sufficiency Question
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How many computers did Michael, a salesman for the computer company Digital Electronics Labs, sell this past year that had more than 4GB of RAM and the Microsoft Windows Vista operating system? (Michael sold no computers with exactly 4GB of RAM)
 40% of the 200 total computers that Michael sold had Vista and less than 4GB of RAM; these computers represent 80% of the total computers that Michael sold with Vista.
 50% of the 200 total computers that Michael sold had Vista; Of the computers that Michael sold without Vista, half had more than 4GB of ram while the other half had less than 4GB of RAM.
Correct Answer: A

Evaluate Statement (1) alone.
 40% of 200 is 80, so Michael sold 80 computers with Vista and less than 4GB of RAM.
 These 80 computers represent 80% of the total computers Michael sold with Vista, so Michael sold a total of 100(=80/80%) computers with Vista.
 Vista_{total} = Vista_{<4GB} + Vista_{>4GB}
100 = 80 + Vista_{>4GB}
Vista_{>4GB} = 20  Statement (1) alone is SUFFICIENT.
 Note: At this point, you should not continue making calculations since you have determined sufficiency. However, to be complete, we included additional information you can deduce:
Vista_{Total} + NoVista_{Total} = Total
100 + NoVista_{Total} = 200
NoVista_{Total} = 100 and Michael sold a total of 100 computers without Vista.
Vista No Vista Total > 4GB RAM 20 < 4GB RAM 80 Total 100 100 200

Evaluate Statement (2) alone.
 "50% of the 200 total computers that Michael sold had Vista" means that 100 computers had Vista. Consequently, 100 computers Michael sold did not have Vista.
 "Of the computers that Michael sold without Vista, half had more than 4GB of RAM while the other half had less than 4GB of RAM" means that onehalf of the 100 computers without Vista (i.e., 50) had more than 4GB of RAM while the other half (i.e., 50) had less than 4GB of RAM.
 Put this information into a table:
Vista No Vista Total > 4GB RAM ?= 50 < 4GB RAM 50 Total 100 100 200  We cannot determine the number of computers sold both with Vista and more than 4GB of RAM, so Statement (2) is NOT SUFFICIENT.
 Statement (2) alone is NOT SUFFICIENT.
 Since Statement (1) alone is SUFFICIENT and Statement (2) alone is NOT SUFFICIENT, answer A is correct.
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