In the figure, find the four angles A, B, C, D in the parallelogram ABCD.

4a + 5a + 6a = 180^o (angle sum property of triangle)
15a = ​180^o
a = 12
4a = 4 x 12 = 48
5a = 5 x 12 = 60
6a = 6 x 12 = 72
angle B = angle D (opposite sides of parallelogram)
72^o = ​angle D
angle D + angle A = 180^o (adjacent angles of parallelogram )
72 + ​angle A​ = 180^o
angle A = 108^o
angle A = angle C
108^o = ​angle C
Therefore,
Angle A = 108^o
Angle B = 72^o
Angle C = 108^o
Angle D = 72^o
​