Two angles of a triangle are in the ratio 4 : 5.

Let the common multiple of all the three angles be $\mathrm{x}$.

Then, the first angle will be $4 \mathrm{x} .$

And the second angle will be $5 \mathrm{x}$.

In a triangle, sum of all the three angles will be equal to $180^{\circ}$.

$\therefore T$ hird angle $=180-(4 x+5 x)=180-9 x$

$\therefore 4 x+5 x=180-9 x$

$\Rightarrow 9 x=180-9 x$

$\Rightarrow 9 x+9 x=180$

$\Rightarrow 18 x=180$

$\Rightarrow x=\frac{180}{18}=10$

First angle $=4 x=4 \times 10=40^{\circ}$

Second angle $=5 x=5 \times 10=50^{\circ}$

Third angle $=4 x+5 x=9 x=9 \times 10=90^{\circ}$