Answer:
So the conclusion is that in presence of air net force acting downward reduces for feather and hence falls slower than coin. But in absence of air resistance, net downward force is just equal to force due to gravity which is same for both coin and feather and hence they fall down at the same rate.
#1
As we know that

now plug in all data into this


now from the formula of strain




#2
As we know that
pressure * area = Force
here we know that


now force is given as

#3
As we know that density of water will vary with the height as given below

here we know that


now density is given as


#4
as we know that pressure changes with depth as per following equation

here we know that

now we will have



here we will have

so it is 20.1 m below the surface
#5
Here net buoyancy force due to water and oil will balance the weight of the block
so here we will have




so it is 3.48 cm below the interface
the answer wil be 2.36+3.38+0.355+1.06=7.155