The speed of the boat is given by the gradient of the line.
Given two points (x1,y1) and (x2,y2) on a graph, the gradient of the line that passes through the two points is given by
In this case,
the line passes through the points (0,0) and (2,2).
We can set (x1,x2) = (0,0) and (y1,y2) = (2,2)
Therefore,
Given that a body travels with speed, s, and in time, t, the distance travelled, d, is given by
In this case,
s = 1km/h, and t = 15hours
Therefore,
Therefore, the boat travelled a distance of 15km