A car traveling at a certain constant speed takes 15 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 48 kilometers per hour. At what speed, in kilometers per hour, is the car traveling? Choose the correct answer:
A) 39  B) 40  C) 41  D) 42  E) 43

Rationale: "48 * t = 1 km = > t = 1 / 48 km / h v * ( t + 15 / 3600 ) = 1 v ( 1 / 48 + 15 / 3600 ) = 1 v ( 90 / 3600 ) = 1 v = 40 km / h the answer is b ."
Answer: B