198. From the fact that $p$ and $q$ are coprime, using the Euclidean algorithm, it is established that any integer $n$ can be represented in the form:

$$n=p x+q y, \text { where } x \text { and } y \text { are integers. }$$

Indeed, let $p>q$; then $p=q d+r$, where $00$ (since the number $n$ is positive). On the plane with coordinates $x_{0}, y_{0}$, the lines $x_{0}=q, y_{0}=0, p x_{0}+$ $+q y_{0}=0$ outline a shaded triangle as shown in Fig. 22; we need to find the number of "integer points" in this triangle. It is clear that this number is exactly half the number of "integer points" in the rectangle $O A B C$ (there are no "integer points" on the diagonal of the rectangle, since $p$ and $q$ are coprime). Since the number of "integer points" inside the rectangle is obviously $(p-1) \cdot(q-1)$, we arrive at the same value of the desired number $t$ as above.