Textbook Problem

In each of 7—14, the relation R is an equivalence relation on the set A. Find the distinct equivalence classes of R.
14. A is the set of all strings of 0’s, l’s, and 2’s that have length 4 and for which the sum of the characters in the string is less than or equal to 2. R is defined on A as follows: For every $s,t\in A,sRt\iff$, the sum of the characters of s equals the sum of the characters of t.