Two adjacent angles on a straight line are in the ratio 5 : 4.

Let the two adjacent angles be 5x and 4x, respectively.

Then,

$5 x+4 x=180^{\circ}$

$\Rightarrow 9 x=180^{\circ}$

$\Rightarrow x=20^{\circ}$

Hence, the two angles are $5 \times 20^{\circ}=100^{\circ}$ and $4 \times 20^{\circ}=80^{\circ}$.