$$\frac{m_1}{m_2} = \frac{V_2}{V_1}$$
$$m_1 = \frac{m_2 \cdot V_2}{V_1}$$
$$m_1 \cdot V_1 = m_2 \cdot V_2$$
$$m_2 = \frac{m_1 \cdot V_1}{V_2}$$
Ответ: $$m_2 = \frac{m_1 \cdot V_1}{V_2}$$