Answer:
[tex]\boxed{\text{10 \%}}[/tex]
Explanation:
The formula for efficiency is Â
[tex]\begin{array}{rcl}\text{Efficiency} & = & \dfrac{\text{useful energy out}}{\text{energy in}} \times 100 \,\% \\\\\eta & = & \dfrac{w_{\text{out}}}{w_{\text{in}} } \times 100 \,\%\\\end{array}[/tex]
Data:
Useful energy = Â 3 J
Energy input  = 30 J
Calculation:
[tex]\begin{array}{rcl}\eta & = & \dfrac{\text{3 J}}{\text{30 J}} \times 100 \,\%\\\\\eta & = & 10 \, \%\\\end{array}\\\text{ The efficiency is }\boxed{\textbf{10 \%}}[/tex]
