Find positive numbers $a$ and $b$ so that the change of variables $s = ax$, $t = by$ transforms the integral $\int \int_R dz dy$ into $\int \int_T \left| \frac{\partial(x, y)}{\partial(s, t)} \right| ds dt$ for the region $R$, the rectangle $0 \le x \le 15$, $0 \le y \le 25$ and the region $T$, the square $0 \le s, t \le 1$.