3.9 A crystal, during the formation stage, uniformly increases its mass. Observing the formation of two crystals, it was noticed that the first one gained the same mass increment over 3 months as the second one over 7 months. However, after a year, it turned out that the first crystal increased its initial mass by $4 \%$, and the second by $5 \%$. Find the ratio of the initial masses of these crystals.

## Trade and monetary relations
3.9 Let the annual increase in mass $x$ be $a$; then, according to the condition, the annual increase in mass $y$ is $\frac{3 a}{7}$. We have $a=0.4 x, \frac{3 a}{7}=0.5 y \Rightarrow \frac{4}{5} \cdot \frac{x}{y}=\frac{7}{3} \Rightarrow x: y=35: 12$.

Answer: $35: 12$.