Answer:
The temperature of cold reservoir should be 246.818 K for efficiency of 35%
Explanation:
In first case we have given efficiency of Carnot engine = 26 % = 0.26
Temperature of cold reservoir 
We know that efficiency of Carnot engine is given by



For second Carnot engine efficiency is given as 35% = 0.35
And temperature of hot reservoir is same so 
So 

So the temperature of cold reservoir should be 246.818 K for efficiency of 35%
true if you are refering to the desing of the experimnt as it does identify the variable
How would you describe the behavior of particles in a solid?
Yes a kg of hydrogen will have more atoms than a kg of lead, because lead has a higher atomic mass, than hydrogen so it will take more atoms of hydrogen to make a kg than lead