Slot method
1st slot, 5 options
2nd slot, 4 options (1 gone to the first slot)
3rd slot, 3 options (1 gone again)
4th slot,2 options (1 less)
multiply
5*4*3*2=120
answer is 120 arrangements
Answer:
V = 20.2969 mm^3 @ t = 10
r = 1.692 mm @ t = 10
Step-by-step explanation:
The solution to the first order ordinary differential equation:

Using Euler's method

Where initial droplet volume is:

Hence, the iterative solution will be as next:
- i = 1, ti = 0, Vi = 65.45

- i = 2, ti = 0.5, Vi = 63.88

- i = 3, ti = 1, Vi = 62.33

We compute the next iterations in MATLAB (see attachment)
Volume @ t = 10 is = 20.2969
The droplet radius at t=10 mins

The average change of droplet radius with time is:
Δr/Δt = 
The value of the evaporation rate is close the value of k = 0.08 mm/min
Hence, the results are accurate and consistent!
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