Mechanical energy equals the sum of potential and kinetic energy. During the process, all PE converts into KE, assuming air resistance is neglected. So, the mechanical energy does not change and is equal to the initial potential energy.
ME
=mgh
=0.005 x 9.81 x 3
=0.147J
X + y = 5
x = 5 - y
2x - y = 7
2(5-y) - y = 7
10 - 2y - y = 7
10 - 3y = 7
10 - 7 = 3y
3 = 3y
y = 1
x + y = 5
x + 1 = 5
x = 5 - 1
x = 4
The maximum efficiency is 10%
Explanation:
The maximum efficiency of a machine is given by:

where
is the temperature of the hot reservoir
is the temperature of the cold reservoir
For the machine in this problem, we have:
is the temperature of the hot reservoir
is the temperature of the cold reservoir
substituting, we find the max efficiency:

So, the efficiency is 0.10 (10%).
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