Part a
Answer: 
For the arrangement considered in part a, all the atoms are aligned side by side along the side of the square surface.
Along one side of the square, the number of atoms that could fit 
This is along one side. On the entire surface:
Number of atoms 
Part b
Answer: 
consider the attached figure below:
The next layer of atoms are filled in the depressions of the first layer. The vertical distance between the two atoms would change. Consider an equilateral triangle drawn by joining the centers of the three atoms.
The atoms along the horizontal side would be aligned side by side; same as above 
The number of atoms along vertical side would vary. vertical distance between two atoms can be calculate using equilateral triangle as shown below.
an equilateral triangle has all the angles 
Let the vertical distance be y.
Then, 
where r is the radius of each atom. 

The number of atoms along the vertical side 
Total number of atoms in this kind of arrangement 
Answer:
This could be done if a stop watch is used to calculate the time taken to hear the echo and a rule should be used to calculate the distance between the bricks and the wall. Then divide distance by time
Explanation:
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Answer:
the answer is The pneumatic mechanical device can only be used as a de-icing device.
Explanation:
An ice protection system prevents the formation of ice, or enables the aircraft to shed the ice before it can grow to a dangerous thickness. Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft surfaces such as wings, propellers and engine intakes.
The pneumatic mechanical device is the Pneumatic deicing boots which was invented by the Goodrich Corporation in 1923. The pneumatic boot is usually made of layers of rubber, with one or more air chambers between the layers.
Any design which utilizes either a mechanical means of breaking the bond of ice to the surface, or which operates on a periodic cycle, is necessarily a de-ice system.