Striver wrote:How much time did it take a certain car to travel 400 kilometers?
(1) The car traveled the first 200 kilometers in 2.5 hours.
(2) If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.
Statement 1: The car traveled the first 200 kilometers in 2.5 hours.
No information about the last 200 kilometers.
INSUFFICIENT.
Statement 2: If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.
Since one speed is 20 kilometers greater than the other, the two speeds are likely to be factors of 400 that are 20 apart: 80 and 100.
Time to travel 400 kilometers at a speed of 80 miles per hour = 400/80 = 5.
Time to travel 400 kilometers at a speed of 100 miles per hour = 400/100 = 4 hours.
Time difference = 5-4 = 1 hour. Success!
No other combination of speeds will work.
To illustrate, if the speeds are 5 and 25, the difference in time = 400/5 - 400/25 = 80-16 = 64 hours.
Thus, the only speed that satisfies statement 2 is 80 kilometers per hour.
SUFFICIENT.
The correct answer is
B.
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