Practice GMAT Data Sufficiency Question
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Police suspected that motorists on a stretch of I75 often exceeded the speed limit yet avoided being caught through the use of radar detectors and jammers. Officer Johnson of the State Police recently pulled over a driver on I75 and accused him of breaking the 50 mileperhour speed limit. Is Officer Johnson’s assertion correct?
 Officer Johnson noted that the driver had traveled 30 miles from point A to point B on I75.
 Officer Johnson noted that it took the driver 30 minutes to travel from point A to point B on I75.
Correct Answer: C
 The basic equation necessary for solving this problem is:
Distance = Rate*Time
D = RT
If Officer Johnson can prove that R, the driver’s average rate or speed, exceeded 50 milesperhour, Officer Johnson can prove that the driver broke the speed limit. We must be able to find R in order to definitively answer the question of whether Officer Johnson’s assertion is correct. 
Evaluate Statement (1) alone.
 Statement (1) indicates that D = 30 miles. Without information about T or R, we cannot find the value of R and, as a result, we cannot prove or disprove Officer Johnson's claim.
 Statement (1) alone is NOT SUFFICIENT.

Evaluate Statement (2) alone.
 Statement (2) indicates that T = 30 minutes. Without information about D or R, we cannot find the value of R and, as a result, we cannot prove or disprove Officer Johnson's claim.
 Statement (2) alone is NOT SUFFICIENT.

Evaluate Statements (1) and (2) together.
 From Statement 1: D = 30 miles
 From Statement 2: T = 30 minutes
 Putting the information together, we can construct the following algebraic equation:
D = RT
30mil = R(30min)
R = 1mil/1min = one mile per minute
R = One mile per minute * 60 minutes per hour = 60 miles per hour.  Since we have a value for R, we can definitively judge the veracity of Office Johnson's claim.
 Statements (1) and (2), when taken together, are SUFFICIENT.
 Since Statement (1) alone is NOT SUFFICIENT and Statement (2) alone is NOT SUFFICIENT, but Statements (1) and (2) when taken together are SUFFICIENT, answer C is correct.
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