Answer:
[tex]8.0\; \rm J[/tex].
Explanation:
The efficiency of a machine is the ratio between the useful output and the energy input:
[tex]\begin{aligned}\text{efficiency} &= \frac{\text{useful output}}{\text{energy input}} \times 100\%\end{aligned}[/tex]
Rearrange this equation to find energy input in terms of efficiency and useful output:
[tex]\displaystyle \text{energy input} = \frac{\text{useful output}}{\text{efficiency} / (100\%)}[/tex].
Substitute in the values: [tex]\text{useful output} = 1.2\; \rm J[/tex] and [tex]\text{efficiency} = 15\%[/tex]. Evaluate to find the value of [tex]\text{energy input}[/tex]:
[tex]\begin{aligned} \text{energy input} &= \frac{\text{useful output}}{\text{efficiency} / (100\%)} \\ &= \frac{1.20\; \rm J}{15\% / (100\%)} \\ &= 8.0\; \rm J\end{aligned}[/tex].
(Rounded to two significant figures as in the value of efficiency.)
