If we take these as coordinates on a graph, where y is the depth and x is the time, we calculate our slope or rate of change where (x1, y1) = (10, 30) and (x2, y2) = (40, 100)

Rate of change = (y2 - y1)/(x2 - x1)

Rate of change = (100 - 30)/(40 - 10)

Rate of change = 70/30

Rate of change =

**2.333 feet per second**