Y=78
A polygon has a total of 5 interior angles
In order to find the total amount of angles in a shape you do (the amount of angles - 2)x180 degrees
(5-2(3))180
3x180=540
Add the numbers you know (10+20+30+40+50=150)
540-150=390 and there are 5y altogether so divide 390 by 5 which is 78
