From the above figure, QS = QR + RS. Let QR = x unit. So,
[tex]\begin{gathered} QS=QR+RS \\ \Rightarrow15=x+\frac{1}{3}x \\ \Rightarrow15=\frac{4}{3}x \\ \Rightarrow45=4x \\ \Rightarrow\frac{45}{4}=x \end{gathered}[/tex]So, we get QR = 45/4 units.
Now, RS = 1/3 QR. So,
[tex]\begin{gathered} RS=\frac{1}{3}QR \\ \Rightarrow RS=\frac{1}{3}\times\frac{45}{4} \\ \Rightarrow RS=\frac{15}{4} \end{gathered}[/tex]Thus, RS = 15/4 units.
