Answer for a)
the diagram would be something like this :
make a meter scale and on the last end place the weight of 20 gf. At 58 cm mark
the fulcrum and at 50 cm mark the weight of scale (downward direction both can be depicted by arrows.
Answer for b)
sum of anticlockwise force= sum of clockwise force
hence let the weight of meter scale be x
x * distance from fulcrum( here 8 cm) = 20 * 42