Multiple choice general knowledge math & puzzles

A train travels at the rate of 10 miles/hr for the first hour of a trip, at 20 miles/hr for the second hour, at 30 miles/hr for the third hour and so on. How many hours will it take the train to complete a 450-mile journey? Assume that the train makes no intermediate stops.

  1. 8

  2. 8.5

  3. 9

  4. 9.5

  5. 10

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Hourly speeds: 10, 20, 30, ... mph (arithmetic progression). Total distance = sum of first n terms = n(n+1)/2 × 10 = 5n(n+1). For 450 miles: 5n(n+1) = 450, n(n+1) = 90, n² + n - 90 = 0, (n+10)(n-9) = 0. Positive integer: n = 9 hours. Option C is correct.