Extract the given data for each firm.

For Firm A:
Number of employees ($n_A$) = 100
Average wage ($\bar{x}_A$) = Rs 3200
Variance ($\sigma_A^2$) = 625

For Firm B:
Number of employees ($n_B$) = 150
Average wage ($\bar{x}_B$) = Rs 2800
Variance ($\sigma_B^2$) = 729

Standard deviation is the square root of the variance.
For Firm A:

$$\sigma_A = \sqrt{\sigma_A^2} = \sqrt{625} = 25$$

For Firm B:

$$\sigma_B = \sqrt{\sigma_B^2} = \sqrt{729} = 27$$

For Firm A:

$$CV_A = \frac{\sigma_A}{\bar{x}_A} \times 100\% = \frac{25}{3200} \times 100\% = 0.0078125 \times 100\% = 0.78125\%$$

For Firm B:

$$CV_B = \frac{\sigma_B}{\bar{x}_B} \times 100\% = \frac{27}{2800} \times 100\% = 0.009642857 \times 100\% \approx 0.9643\%$$

Since $CV_B \approx 0.9643\%$ is greater than $CV_A \approx 0.78125\%$, Firm B has greater variability in individual wages.

b) To calculate the average monthly wage and the variance of wages of all workers from both firms combined.

The formula for the combined mean is:

$$\bar{x}_{combined} = \frac{n_A \bar{x}_A + n_B \bar{x}_B}{n_A + n_B}$$

Substitute the values:

$$\bar{x}_{combined} = \frac{(100 \times 3200) + (150 \times 2800)}{100 + 150}$$ $$\bar{x}_{combined} = \frac{320000 + 420000}{250}$$ $$\bar{x}_{combined} = \frac{740000}{250}$$ $$\bar{x}_{combined} = 2960$$

The combined average monthly wage is Rs 2960.

The formula for combined variance is:

$$\sigma_{combined}^2 = \frac{n_A(\sigma_A^2 + d_A^2) + n_B(\sigma_B^2 + d_B^2)}{n_A + n_B}$$

where $d_A = \bar{x}_A - \bar{x}_{combined}$ and $d_B = \bar{x}_B - \bar{x}_{combined}$.

First, calculate the differences ($d_A$ and $d_B$):

$$d_A = 3200 - 2960 = 240$$ $$d_B = 2800 - 2960 = -160$$

Next, calculate the squared differences ($d_A^2$ and $d_B^2$):

$$d_A^2 = (240)^2 = 57600$$ $$d_B^2 = (-160)^2 = 25600$$

Now, substitute these values into the combined variance formula:

$$\sigma_{combined}^2 = \frac{100(625 + 57600) + 150(729 + 25600)}{100 + 150}$$ $$\sigma_{combined}^2 = \frac{100(58225) + 150(26329)}{250}$$ $$\sigma_{combined}^2 = \frac{5822500 + 3949350}{250}$$ $$\sigma_{combined}^2 = \frac{9771850}{250}$$ $$\sigma_{combined}^2 = 39087.4$$

The combined variance of wages is 39087.4.

a) Firm with greater variability: $\boxed{\text{Firm B}}$

b) Combined average monthly wage: $\boxed{\text{Rs 2960}}$
Combined variance of wages: $\boxed{\text{39087.4}}$