The average salary of male employees in a firm was $\text{Rs. }52$ and that of female employees was $\text{Rs. }42$. If the mean salary of all the employees was $\text{Rs. }50$, what was the percentage of female employees?

Let the number of male employees be $M$ and female employees be $F$.
Using the method of Allegation:

* Average salary of males = $52$
* Average salary of females = $42$
* Mean salary of all employees = $50$

$$\text{Ratio of Male to Female }(M:F)=(50-42):(52-50)$$

$$M:F=8:2=4:1$$

Percentage of female employees:

$$\text{Percentage of Females}=\frac{F}{M+F}\times 100\%$$

$$\text{Percentage of Females}=\frac{1}{4+1}\times 100\%=\frac{1}{5}\times 100\%=20\%$$