Solution :
Given
[tex]$W_1$[/tex] = weight of the first side
= 6 kg = 6000 g
[tex]$D_1$[/tex] = distance of the first weight from the triangle.
= 3 m = 300 cm
[tex]$D_2$[/tex] = distance of the first weight from the triangle.
= 600 cm
Let, [tex]$W_2$[/tex] = weight of the second side
So, [tex]$W_1$[/tex] x [tex]$D_1$[/tex] = [tex]$W_2$[/tex] x [tex]$D_2$[/tex]
or [tex]$W_2=\frac{W_1 \times D_1}{D_2}$[/tex]
[tex]$W_2=\frac{6000\times 300}{600}$[/tex]
= 3000 g
Therefore, the quantity that is used to replace the question mark in order to make the scale balance is 3000 g.