In a parallelogram, an angle is $\dfrac{4}{5}$ th of its adjacent angle, then angles of the parallelogram are :

1. 80°, 100°, 80°, 100°

2. 70°, 110°, 70°, 110°

3. 60°, 120°, 60°, 120°

4. none of these

In a parallelogram, adjacent angles are supplementary.

Let one of the angle of parallelogram be x.

The adjacent angle is given as $\dfrac{4x}{5}$.

$\Rightarrow x + \dfrac{4x}{5} = 180° \\[1em] \Rightarrow \dfrac{5x + 4x}{5} = 180° \\[1em] \Rightarrow \dfrac{9x}{5} = 180° \\[1em] \Rightarrow 9x = 180° \times 5 \\[1em] \Rightarrow 9x = 900° \\[1em] \Rightarrow x = 100°.$

The adjacent angle = $\dfrac{4 \times 100°}{5}$ = 80°.

The angles opposite to 100° is also 100°.

The angle opposite to 80° is also 80°.

Hence, option 1 is the correct option.