An 80-kg man climbs a 6-m ladder in 12 seconds. what is his average power expended?

If the ladder is vertical, then the work done in ascending equals his increase in potential energy. 
For a man of mass (m) climbing height (h), the PE is (mgh), where g is the acceleration due to gravity. 

If he does this in time t, his average power (rate of working) is: 
mgh / t 
= 80 * 9.81 * 6 / 12 
= 392.4 W. 

If the ladder is not vertical but inclined at an angle the horizontal, the power is mgh sin(a) / t.