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)