Using the percentile definition 1.3.5, the 25th percentile is computed by considering

\begin{equation*} (n+1)s = (4+1)0.25 = 5/4 = 1.25\text{.} \end{equation*}

So, m = 1 and r = 0.25. Therefore

\begin{equation*} P^{0.25} = 0.75 \times 2 + 0.25 \times 5 = 2.75 \end{equation*}

as noted above.