forces question

A dart of mass 22g is travelling at 7ms-^1 when it strikes a board. How thick must the board be to prevent the point of the dart reaching the wall behind the board? Assume the resistive force exerted by the material of the board is constant and of value 20N. **Please note that this question has already been answered (see attached word doc). I am posting again here as I can’t get my head around where the 0.067m/s comes from in the beginning**

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