<span>Objective Lenses: Usually you will find 3 or 4 objective lenses on a microscope. They almost always consist of 4X, 10X, 40X and 100X powers. When coupled with a10X (most common) eyepiece lens, we get total magnifications of 40X (4X times10X), 100X , 400X and 1000X.</span>
Answer:

So then the difference of temperature across the material would be 
Explanation:
For this case we can use the Fourier Law of heat conduction given by the following equation:
(1)
Where k = thermal conductivity = 0.2 W/ mK
A= 1m^2 represent the cross sectional area
Q= 3KW represent the rate of heat transfer
is the temperature of difference that we want to find
represent the thickness of the material
If we solve
in absolute value from the equation (1) we got:

First we convert 3KW to W and we got:

And we have everything to replace and we got:

So then the difference of temperature across the material would be 
The mass of magnesium oxide must be 40 grams
Explanation:
The law of conservation of mass states that the total mass of the reactant in a chemical reaction must be equal to the total mass of the products after the reaction has completed.
In this problem, we have:
Reactants:
Magnesium metal (mass: 24 grams)
Oxygen gas (mass: 16 grams)
Products:
Magnesium oxide (mass: ?)
According to the law of conservation of mass, the mass of the magnesium oxide must be equal to the sum of the mass of the magnesium metal and the oxygen gass. Therefore:
Mass (magnesium oxide) = 24 g + 16 g = 40 g
Learn more about law of conservation of mass:
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