Your pictures aren't lettered, but it's the pic with the line intersecting with 40 on the y axis (40 meters (in thousands)) and 50 on the x axis (50 hours).
The distance swum in 10 hours is approximately 8000 meters, so it will take 10000/8000 = 1.25 hours more to swim 10000 meters. This can be represented by having a slightly steeper gradient, which is present in the second graph.
