here we need to balance the torque due to weight of boy with the torque due to weight of girl on the other side
so here we can say
(weight of boy) x (his distance from fulcrum) = (weight of girl) x (her distance from fulcrum)
[tex](30\times 9.8)\times (2 m) = (45 \times 9.8) \times r_2[/tex]
[tex]60 = 45 r_2[/tex]
[tex]r_2 = \frac{60}{45}[/tex]
[tex]r_2 = 1.33 m[/tex]
so girl distance from fulcrum must be 1.33 m