29. $$\frac{x+5}{x+15} = \frac{3}{4}$$.

$$\begin{aligned}&\frac{x+5}{x+15} = \frac{3}{4}\\ &4(x + 5) = 3(x + 15)\\ &4x + 20 = 3x + 45\\ &4x - 3x = 45 - 20\\ &x = 25\end{aligned}$$ Ответ: $$\boxed{x = 25}$$