A frequency is a count, and a frequency tree shows how one total is split into categories such as boys and girls or tea and coffee. The first node is usually the grand total, and each new split breaks a group into smaller parts.

At every split, the two child numbers add back to the parent number. If one child is missing, use the following calculation to find the value:

$$missing\; child=parent\; total-known\; child$$

For example, if $90$ students are split into set A and set B, and $34$ are in set A, then set B has $90-34=56$. This same add-then-subtract idea is the backbone of every frequency tree question.