Calculate the velocity ratio, weight of cement bag, mechanical advantage, and efficiency of the pulley system shown in Figure 4, given the effort is 300 N and the cement bag's mass is 50 kg.

In Figure 4, there are 3 rope segments supporting the lower block (which carries the load).

The velocity ratio (VR) is the number of rope segments supporting the movable load.

The velocity ratio of the machine is $\boxed{\text{3}}$.

Mass ($m$) = 50 kg
Acceleration due to gravity ($g$) $\approx 10 \text{ m/s}^2$ (standard approximation for such problems unless specified otherwise).

The weight of the bag of cement is $\boxed{\text{500 N}}$.

Load ($L$) = Weight of the bag of cement = 500 N (from part b)
Effort ($E$) = 300 N (given in the problem description)

The mechanical advantage of the pulley system is $\boxed{\text{1.67}}$.

MA = 1.67 (from part c)
VR = 3 (from part a)

The efficiency of the pulley system is $\boxed{\text{55.67\%}}$.