Question

if a runner's power is 400 w as she run's how much chemical energy does she convert into other forms in 10.0 minutes?

Answer 1

The runner runs for a total time of
$\Delta t=10.0 min= 600 s$
The energy converted by the runner during this time is equal to the power of the runner times the total time:
$E=P \Delta t=400 W \cdot 600 s =2.4 \cdot 10^5 J$