A transit company operates commuter trains to help people get to the city each day. Each train travels on a straight route to and from the farthest station, stopping at other stations along the route.

The graph shows the distance y miles the train is from the station after x minutes.

Type your answer in the box. You may use numbers, a decimal point (.), and/or a negative sign (-) in your answer.

Based on the graph, what was the average speed, in miles per minute, of the train during the interval of 30 to 40 minutes?