Final answer:
Alex exerted a power of 30 watts (W) while dragging a log across the yard, calculated by dividing the work done (900 J) by the time (30 seconds).
Step-by-step explanation:
The question is asking to calculate the power exerted by Alex when she dragged a log across the yard. Power is defined as the rate at which work is done, which is the amount of work done divided by the time taken to do that work. In this case, Alex did 900 joules (J) of work in 30 seconds. To find the power (P), we use the formula P = work/time.
Power = 900 J / 30 seconds = 30 watts (W)
Therefore, Alex exerted a power of 30 W while dragging the log.