Textbook Problem

In each of 3—6, the relation R is an equivalence relation on A. As in Example 8.3.5, first find the specified equivalence classes. Then state the number of distinct equivalence classes for R and list them.
4. $A=\left\{a,b,c,d\right\}$ $R=\left\{\left(a,a\right),\left(b,b\right),\left(b,d\right),\left(c,c\right),\left(d,\text{}b\right),\left(d,d\right)\right\}$equivalence classes: [a], [b], [c], [d]