Question 350270
Hi,
Let x be one of the angles, (180 - x) the other
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Set up the proportion
{{{3/15 = (x/(180-x))}}}
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cross multiply to solve
15*x = 3*(180-x)
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simplify and solve
 15x = 540 - 3x
18x = 540
 x = 30degrees
Supplement is 150degrees
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check your answer
3/15 = 30/150