Lisa runs around a race track. Her speed is shown over 90 seconds on a speed-time graph below. Between which two times is her speed strictly decreasing and then strictly increasing?
On this graph, the 𝑦-axis represents Lisa’s speed for a duration of time. And the time is on the 𝑥-axis. When the line is going up into the right, Lisa’s speed is increasing. A horizontal line represents constant speed, a speed that is not changing. And a line that is going downward and towards the right is a decreasing speed. We’re looking for a time period where Lisa’s speed strictly decreased and immediately after strictly increased.
From time zero to 15, her speed was increasing. It remained constant from 15 seconds to 30 seconds. Her speed decreased from 30 seconds to 45 seconds, increased from 45 seconds to 60 seconds, remained constant for another 15 seconds, and decreased for the final 15 seconds. We were looking for a decrease followed by an increase. This kind of pattern is found between 30 seconds and 60 seconds.
Lisa’s speed was strictly decreasing and then strictly increasing between 30 and 60 seconds.