The wedge is kept below the 60 cm mark on the metre scale of negligible mass.  Known masses of 1kg and 2kg are hung at the 20 cm and 30 cm mark respectively.  Where will a 4 kg mass be hung on the metre scale to balance it?  

The anti clockwise moments due to 1kg and 2kg are 
= (1kg wt) (60-20)cm + (2kg wt) (60-30) cm
= (1kg wt) (40cm) + (2kg wt) (30 cm) 
= 100 kg wt cm
The clockwise moment due to 
$4kg=4kgwt\times xcm$
$\Rightarrow 100=4xorx=25cm$
So the 4 kg mass must be hung at 
(60 cm + x) = (60 cm + 25 cm) = 85 cm 
Mark to balance the scale.