1 1 15 30 -- --

$$\frac{1}{15} - \frac{1}{30} = \frac{1 \cdot 2}{15 \cdot 2} - \frac{1}{30} = \frac{2}{30} - \frac{1}{30} = \frac{1}{30}$$

Ответ: $$\frac{1}{30}$$