Answer:
The effective mass at the 50 cm mark will be 1.5 kg because the meter stick is uniform and .5 kg can be considered to be acting at 50 cm
3 * (50 - x) = 1.5 * x
Balancing torques where x is the distance from the 50 cm mark
150 - 3 x = 1.5 x
x = 150 / 4.5 = 33.3
x would be at 50 - 33.3 = 16.7
Check: 3 * 16.7 = 1.5 * 33.3
50.1 = 50 within rounding