To solve this problem, let us first remember that the formula for rate is:
rate = f (n) = A (r)^(n-1)
Where,
A = the 1st term
r = fractional rate of evaporation
n = number of terms
Now let us calculate for the fraction rate of evaporation, r. Since it is stated that the evaporation is happening at a fixed rate, therefore we can use any two given f (n) values to find for r. In this case, let us use when n=1 and 2.
r = 27 / 54
r = 0.5
The first term in the series is when n = 1 and that is:
A = 54
Therefore substituting all known values to the equation:
f (n) = 54 (0.5)^(n-1) (ANSWER)
