Answer:
150 g 
Explanation:
From the question;
- Half meter rule (50 cm) is pivoted at 30 cm 
- Mass of 50 g Pivoted at 45 cm 
It means the mass is 15 cm from the pivot
- Mass of the meter rule is always at the center of the rule ( 25 cm mark)
- Therefore, the mass of the half meter rule is experienced 5 cm from the pivot.
- But, According to law of moments, clockwise moments is equal to anticlockwise moments when a system is at a state of balance.
Therefore;
Assuming the mass of half meter rule is m
Then;
m × 5 cm = 50 g × 15 cm 
 m = 150 g 
Therefore;
Mass of the  half meter rule is 150 g