(expect a diagram of a uniform seesaw with a pivot in the middle, infered from the question)

For the seesaw to turn, Bob must create a greater Moment than Ann.

Moment = Force * Perpedicular distance to pivot

Ann's Moment = 600N * 2m = 1200Nm

Calculate the point at which the moments are equal:

Bob's Moment = 800N * Xm = 1200Nm

solve to find X = 1.5m.

Therefore Bob must have travelled 3.5m.

Use of Speed = Distance / Time, rearanged to Time = Distance / Speed, to find time, t:

t = 3.5m / 2m/s = 1.75s.