Hm... interesting little problem if a current was also included...
Well, anyway, the time Grant takes to cover x is given by:
The time Grant takes to cover the distance on the ground, y, is given by:
The distance y is given by:
The total time is:
This all give:
Differentiate with respect to x.
Equate this to zero:
Square both sides, solve for x and you should get 25.92724... = 26 m