There are 21 poles with a constant distance between each pole. A car takes 30 seconds to reach the 16th pole. How long does it take to reach the last pole?

A. 39 seconds
B. 40 seconds
C. 39.38 seconds
D. 39.5 seconds
The first pole is crossed at the zeroth second. The timer starts after the car crosses the first pole. Let the distance between each pole be x m. To reach the 16th pole, the distance travelled by the car is 15x. The speed of the car is 15x/30 = x/2 m/s To reach the 21st pole, it has to travel a distance of 20x. Time taken
= 20x÷(x/2)
= 40 seconds