To find the useful energy generated by the machine, you can use the formula for efficiency:
\[ \text{Efficiency} = \frac{\text{Useful Energy Output}}{\text{Energy Input}} \times 100 \]
In this case, the efficiency is 15% and the energy input is 300 joules. Rearranging the formula to find the useful energy output:
\[ \text{Useful Energy Output} = \text{Efficiency} \times \text{Energy Input} / 100 \]
Substituting the values:
\[ \text{Useful Energy Output} = 15% \times 300 , \text{joules} / 100 \]
\[ \text{Useful Energy Output} = 0.15 \times 300 = 45 , \text{joules} \]
So, the useful energy generated is 45 joules.