80% of an angle is the supplement of 140°. Find the angle.

The supplement of an angle is found by subtracting it from 180°.

∴ 180° − 140° = 40°

Let the required angle be y.

80% of y = 40°

$80\% \text { of } y = 40^{\circ} \\[1em] \Rightarrow \dfrac{80}{100} \times y = 40^{\circ} \\[1em] \Rightarrow \dfrac{4}{5} \times y = 40^{\circ} \\[1em] \Rightarrow 4y = 40^{\circ} \times 5 \\[1em] \Rightarrow 4y = 200^{\circ} \\[1em] \Rightarrow y = \dfrac{200^{\circ}}{4} \\[1em] \Rightarrow y = 50^{\circ}$

Hence, the required angle is 50°.