Find the angle which is double to its supplement 

let one of the angle be x

therefore the other angle = 2x

so ​by the given condition,

2x + x = 180⁰                [as measures of supplementary angles = 180⁰]

that implies 3x= 180⁰

that implies x = 180/3 = 60⁰

therefore the 1st angle = 60⁰

therefore the other angle = 60 X 2= 120⁰

hope this helps.                    [⁰ = degree]

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