2y+20=100
2y=80
y=40
The explanation for this would because because the two base angles of an isosceles triangle are equal so since the both equal to 40 degrees, in total would be 80 degrees.
Since the total degrees of a triangle is 180 and you know the base angles make up for 80 percent you subtract that from the total of the angle your trying to find, which is 2y+20.