In the adjoining figure, y = ?
x + y + 90 = 180 (sum of angles on a straight line)
x + y = 90 ………………….(i)
3x + 72 = 180 (sum of angles on a straight line)
3x = 108
x = 108/3 =36O
Putting this value in eq (i), we get
x + y = 90
36 + y = 90
Y = 90 – 36 = 54O