The monthly salaries of Pia and Som are in the ratio of 5 : 4. From her monthly salary, gives 3 5 to her mother, 15% towards her sister's tuition fees, 18% towards a loan and she shops with the remaining amount, which is Rs. 2100. What is the monthly salary of Som ?

Answer: Option D

Let the monthly salary of Pia and Som be 5a and 4a respectively.
Then,
Money given by Pia to her mother
$$\begin{array}{rl}& =5a\times \frac{3}{5}=3a\end{array}$$
Money given by Pia as sister's tuition fees
$$\begin{array}{rl}& =15\%\text{ of }5a\\ & =\frac{15\times 5a}{100}\\ & =\frac{75a}{100}=0.75a\end{array}$$
Money given by Pia towards loan
$$\begin{array}{rl}& =18\%\text{ of }5a\\ & =\frac{18\times 5a}{100}\\ & =\frac{90a}{100}=0.9a\\ & \therefore \text{Total money given}\\ & =3a+0.75a+0.90a\\ & =4.65a\\ & \therefore \text{Remaining amount}\\ & =5a-4.65\\ & =0.35a\\ & \text{Pia have remaining amount}\\ & =\text{Rs}\text{. }2100\\ & 0.35a=2100\\ & \therefore \frac{35a}{100}=2100\\ & \Rightarrow a=\frac{2100\times 100}{35}\\ & a=\text{Rs}\text{. }6000\\ & \therefore \text{Monthly salary of Som}\\ & =4a=4\times 6000\\ & =\text{Rs}\text{. }24000\end{array}$$