Two supplementary angles are such that one of them is 20° less than three times the other. Find the larger angle.

Let the angles be $x$ and $y$ with $x$ the larger. $x+y=180$ and $x=3y-20$. Substituting: $3y-20+y=180$, so $4y=200,y=50$ and $x=130$. Check: 130 + 50 = 180 and $3\times 50-20=130$. 50° is the smaller angle; 135° comes from dropping the 20 and using $x=3y$; 140° comes from the sign slip $x=3y+20$.