Speed can be defined as the ratio of Change in Distance and Change in Time
i.e
Speed = (Change in distance) / (Change in time)
Since the graph is a straight line, we can consider any interval to calculate the speed as for a Linear graph the change remains the same.
From the graph we can see that at 1 hr the distance covered is 105 miles and at 2 hr the distance covered is 175 miles.
Change in Distance = 175 – 105 = 70 miles
Change in time = 2 – 1 = 1 hour
So, the speed is = 70 mile/1 hour = 70 miles per hour.
So option B gives the correct answer
