Answer:
92.25m
Explanation:
In order to solve the exercise, it is necessary to apply the concept of construtive interference due to a path difference.
The formula is given by,

where,
n is the index of refraction of the medium in which the wave is traveling
wavelenght
is the path difference
m = integer (0,1,2,3...)
Since in this case we are dealing with an atmospheric environment, where air is predominant, we approximate n to 1.
And since we need the reflected wave,

Where x is the distance in one direction without return.
The distance must correspond to the minimum therefore m = 0, so



Then the minimum distance is:



Therefore the minimum distance from the mountain to the receiver that produces destructive interference at the receiver is 92.25m
Answer:
1
The mass of the Potassium-40 is 
2
The Dose per year in Sieverts is 
Explanation:
From the question we are told that
The isotopes of potassium in the body are Potassium-39, Potassium-40, and Potassium-
41
Their abundance is 93.26%, 0.012% and 6.728%
The mass of potassium contained in human body is
per kg of the body
The mass of the first body is 
Now the mass of potassium in this body is mathematically evaluated as

substituting value


The amount of Potassium-40 present is mathematically evaluated as
0.012% * 0.024


The dose of energy absorbed per year is mathematically represented as

Where E is the energy absorbed which is given as 
Substituting value


The Dose in Sieverts is evaluated as



The north and south forces have no effect on the east and west forces.
We can handle the two groups separately.
(20N south) + (30N north)
= (20N south) + (20N north + 10N north)
= 10N north .
(40N east) + (50N west)
= (40N east) + (40N west + 10N west)
= 10N west .