Answer:
Energy is force times distance. For your problem, no matter how long you push, the wall still goes nowhere, so there is no obvious energy transfer. so in conclusion, you actually didn't do anything :(
Explanation:
 
        
                    
             
        
        
        
Answer:
speed of current is 5 mile/hr
Explanation:
GIVEN DATA:
speed of motorboat = 15 miles/hr relative with water
let c is speed of current
15-c is speed of boat at  upstream
15+c is speed of boat at downstream
 we know that 
travel time=distance/speed

150+10c+150-10c=1.5(15-c)(15+c)
300=1.5(225-c^2)
300=337.5-1.5c^2
200=225-c^2
c^2=25
c = 5
so speed of current is 5 mile/hr