Answer
$M=74.4 g$
Work Step by Step
In the balanced state, net torque is zero; that is, leftward torque is equal to rightward torque. Mathematically:
Leftward torque=Rightward torque
$2mg(45.5-12)=Mg(50-45.5)$
$2\times5(33.5)=M(4.5)$
$335=M(4.5)$
or $M=\frac{335}{4.5}=74.4 g$
Thus, the mass of meter stick is $74.4g$
