The Official Guide For GMAT® Quantitative Review, 2ND Edition
How long did it take Betty to drive nonstop on a trip from her home to Denver, Colorado?
(1) If Betty's average speed for the trip had been 3/2 times as fast, the trip would have taken 2 hours.
(2) Betty's average speed for the trip was 50 miles per hour.
We need to know how long it took Betty to drive, so time=distance/speed.
s1 is average speed
t1 is the time needed to complete the journey when travelling at speed s1
s2 is the average speed when travelling at faster speed
t2 is the time needed to complete the journey when travelling at speed s2.
But also as per (1):
or s2=2/3*s1 (equation #1)
t2=d/s2=2hours (from statement 1)
But also as per (eq. #1):
t2= d/s2 = d/((2/3)*s1) = 2 hours --> t2 = (2/3)*(d/s1) = 2 --> (2/3)*t1 = 2 --> t1= 3 hours
Hence,(1) is SUFFICIENT
(2) Nothing is said about distance between "her home" and Colorado, hence we have no means to determine t or d/s.(2) IS INSUFFICIENTANSWER: A