Multiple choice general knowledge math & puzzles

If a car had increased its average speed for a 180-mile trip by 5 mph, the trip would have been completed in 30 minutes less. What was the car's original speed?

  1. 40 mph

  2. 50 mph

  3. 90 mph

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let original speed be $v$. Time taken is $180/v$. With speed $v+5$, time is $180/(v+5)$. The difference is $0.5$ hours. Solving $180/v - 180/(v+5) = 0.5$ gives $v^2 + 5v - 1800 = 0$. Factoring $(v+45)(v-40) = 0$ yields $v=40$. Other options do not satisfy the time difference.

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