Let f(x) be a function defined on set of non negative integers and taking values in the same set such that f satisfies:

x – f(x) = 19[x/19] – 90[f(x)/90] and, 1900< f(1990) < 2000.

Determine the possible value(s) that f(1990) can take.

NOTE: [n] denotes the greatest integer < = n