I believe that in my current, this is meant to be solved using torque counter clockwise is equal to torque clockwise, and I got as far as plugging in the numbers (50gr=100g[0.5]). I know that these numbers are correct because I get the correct solution but coming up with the numbers I'm confused on one key aspect, why do we cut the board length in half and then from the half go out to the edge of the building.

5m 2m 37. A 5-meter uniform plank of mass 100 kilograms rests on the top of a building with 2 meters extended over the edge as shown above. How far can a 50-kilogram person venture past the edge of the building on the plank, if at all, before the plank just begins to tip? [Hint: This is where the board just starts to tilt, so the only force exerted by the building on the board is at the edge.]