the lines through side AB and side AC tell us that both sides are congruent, and by the isosceles triangle theorem that means that m∠ABC = m∠ACB so m∠ABC = 40 Since all 3 angles in a triangle always add up to 180, we know that (40) + (5x) + (2y + 20) = 180 We substitute our known value for 5x and solve for y: 40 + 40 + 2y + 20 = 180 2y + 100 = 180 subtract 100 from both sides 2y = 80 divide both sides by 2 to get our answer: y = 40
