How many walls can this battery-powered crane lift before recharging?

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MichaelTam
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Homework Statement
Exercise
Relevant Equations
PE=mgh,
A new, innovative type of crane is battery powered. Its battery has a total energy capacity U , expressed in Joule. Determine how many walls, N of mass m , the crane can lift before the battery has 20% left and needs to be recharged. The walls need to be lifted from /h_1/ to /h_2/ a new location at height . You may assume that energy is only used when the walls are lifted and you may disregard any energy losses.
 

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My solution is incorrect, but I still can’t find out the error….
 
N=0.8*U/(g*(h_2-h_1)), you may see it at the second post
 
each wall has a mass of m but N is the sum of those mass.
 
But I cannot express N as variable m
 
the last exercise tells me m=1000kg, but I don’t know if I can substitute that into this situation or not.
 
‘That’ means the last exercise it mention m is equal to 1000kg, I think I doesn’t work in this exercise.
 
I use the equation of
1.Potential energy = N*m*g*(h_2-h_1)
2.Energy of the battery can be used = (1-20%)*U
(1) = (2)
Then I find N=N=0.8*U/(m*g*(h_2-h_1)) but how can I express m in terms of other variable?
 
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MichaelTam said:
I use the equation of
1.Potential energy = N*m*g*(h_2-h_1)
2.Energy of the battery can be used = (1-20%)*U
(1) = (2)
Then I find N=N=0.8*U/(m*g*(h_2-h_1))
That's the answer.

MichaelTam said:
but how can I express m in terms of other variable?
I don't understand this question.
 
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I found the system is coming up with a error, the solution can be include m now, I’m sorrry….