Answer:
420J
Explanation:
The formula (useful output ÷ total output) × 100 is used. We need to find the the useful output. So:
(useful ÷ 600) × 100 = 70(%)
We rearrange the formula so that 'useful' is the subject of the equation.
useful ÷ 600 = 0.7 {70 ÷ 100}
useful = 0.7 × 600
useful = 420J
