Let G be the length of the ground, W the length of the walkways in total.

Let T be the amount of time you can run at the higher speed, v’. Let T_G be the portion of that time running on the ground.

Assume that you spend T time running at speed v’ and don’t complete the journey, otherwise obviously you just run at full speed the whole time.

Then the time to complete the journey is simply:

T + (G-v’ * T_G)/v + w – (v’+u)(T-T_G)/(v+u). This is a linear function of T_G and the coefficient of T_G is:

(v’+u)/(v+u) – v’/v which can easily be verified to be negative because v’ > v. Therefore to minimize the time, T_G should be as large as possible (i.e you should run on the ground as long as you can).

]]>More specifically, do exist suitcases S1 and S2, where the linear dimension of S1 is more than the linear dimension of S2, but still S1 fits completely inside S2 (for example diagonally)???

]]>So it is not as easy as you believe..it depends on how long are the walkways and how long the distance between them, and if the point b is just at the end of a walkway or not…

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