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
