Please submit your concern

Explanation:

Let the time taken by P alone be 6x days.
∴ Time taken by Q alone will be 3x days and that by R will be 2x days.

Working together the three of them can finish the work in 5 days.

⇒ $\frac{1}{5}=\frac{1}{\mathrm{P}}+\frac{1}{\mathrm{Q}}+\frac{1}{\mathrm{R}}$

⇒ $\frac{1}{5}=\frac{1}{6x}+\frac{1}{3x}+\frac{1}{2x}$

⇒ $\frac{1}{5}=\frac{1+2+3}{6x}$

⇒ $\frac{1}{5}=\frac{1}{x}$

⇒ x = 5

∴ Q alone can complete the work in 3x = 15 days.

Hence, option (c).