\(1h=60min\\\\\begin{array}{ccc}60min&-&\$11\\\\15min&-&\$x\end{array}\\\\\\cross\ multiply:\\\\60x=11\cdot15\ \ \ /:60\\\\x=\frac{11\cdot15}{60}\\\\x=\frac{11}{4}\\\\x=2.75\ (\$)\leftarrow Answer\)
\(other\ method:\\\\\$11\ -\ 60min\ \ \ \ /:4\\\\\$11:4\ -\ 60min:4\\\\\$2.75\ -\ 15min\)
\(Answer:a\ cost\ \frac{\$2.75}{15min}\)