Hi All,
This question is based on a math 'truism' that many Test Takers don't know: Increasing your speed by 50% decreases the amount of travel time by 1/3. Here's why:
Distance = (Rate)(Time)
D = (R)(T)
Since the distance remains the same (you're just changing the rate and time), any increase in rate or time is met with a change in the other term. Increasing the rate by 50% would give us...
D = (3/2)(R)(?)
The change in time has to 'offset' the (3/2) that is now part of the calculation. The fraction (2/3) creates that 'offset', which gives us....
D = (3/2)(R)(2/3)(T)
D = (6/6)(R)(T)
In this prompt, we're told that the time is decreased by 1/3 after the 15 miles/hour increase in speed. We're asked for the ORIGINAL speed, which means that the 15 miles/hour represents the 50% increase. Thus, the original speed must be 30 miles/hour.
Final Answer:
GMAT assassins aren't born, they're made,
Rich