In data interpretation, if a graph shows a straight line with a slope of 3 units per second, what rate does the slope describe?

• A. 3 seconds per unit
• B. 3 units per second
• C. 1 unit per 3 seconds
• D. 0 units per second

Answer: (B)

On a graph, slope represents the rate of change: how much the vertical quantity changes for a given amount of horizontal change. If the horizontal axis is time in seconds and the vertical axis is a quantity measured in units, a slope of 3 units per second means the quantity increases by 3 units for every 1-second increase in time. So the rate described is 3 units per second. For example, after 2 seconds the quantity would be about 6 units higher, and after 5 seconds about 15 units higher, assuming the line continues.

The other expressions describe different rates: 3 seconds per unit would be the reciprocal of the given slope, meaning only 1/3 as many units per second; 1 unit per 3 seconds is also 0.333 units per second, and 0 units per second would indicate no change at all.