Answer: 
Explanation:
Given
Distance between two speakers is 8 m
Man is standing 12 away from the wall
When the person moves 3 parallel to the wall
the parallel distances from the speaker become 4+3, 4-3
Now, the difference of distances from the speaker is

Condition for destructive interference is

for second destructive interference; n=2

The work done is 400 J
Explanation:
The work done by you in pushing the box along the slope is given by

where
F is the magnitude of the force applied
d is the distance covered by the box along the slope
Here we have the following:
F = 200 N is the magnitude of the force applied
is the distance covered, where
h = 1 m is the vertical rise
is the slope of the plane
Substituting and solving, we find

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L = illuminance
A = surface
i = intensity
L = i / A ==: i = L * A
i = 6 lux * 4 m^2 = 24 lumen
It's a chemical change since it cannot be reversed and the saliva releases a new substance