$*)\begin{cases}(CH_3)_2NH:x mol \\C_\overline{n} H_{2\overline{n}+2-2a }:y mol\\V_X=100 ml\\V_{H_2O}=300 ml\\V_{CO_2}+V_{N_2}=250 ml \end{cases}$$\Rightarrow (x+y):[(2x+0,5x)+\overline{n}y]:[3,5x+y(\overline{n}+1-a)]=100:250:300$ $=1:2,5:3$
$\Rightarrow \begin{cases}2,5(x+y)=(2,5x+\overline{n}y) \\ 3(x+y)=3,5x+y(\overline{n}+1-a) \end{cases}\Rightarrow \begin{cases}\overline{n} =2,5 \\x=y(1-a) \end{cases}\Rightarrow\begin{cases}\overline{n} =2,5 \\ a=0 \end{cases}$
$\Rightarrow \begin{cases}C_2H_6 \\C_3H_8 \end{cases}$