Hello el123With more data, you could plot a graph, and draw tangents to the graph at the points where you wanted the growth rate. However, with this small amount of data, you can only work out average rates - there's not really enough data to plot a graph.

In the first minutes the length has increased by mm. So the average rate during this period is mm per minute.

In the second minutes, the length has increased by ...? So the average growth rate is ...?

And, overall, in minutes the average growth rate is ... ?

Can you complete the calculations?

Grandad