Let ∠A and ∠B are two adjacent angles.
But we know that sum of adjacent angles of a parallelogram is 180o
∠A + ∠B = 180o
Given that adjacent angles of a parallelogram are in the ratio 4:5 and let that ratio be multiple of x
∠A + ∠B = 180o
4x + 5x = 180o
9x = 180o
x = 180/9
x= 20o
∠A = 4x = 4 × 20 = 80o
∠B = 5x = 5 × 20 = 100o
Also ∠B + ∠C = 180o [Since ∠B and ∠C are adjacent angles]
100o+ ∠C = 180o
∠C = 180o– 100o= 80o.
Now ∠C + ∠D = 180o [Since ∠C and ∠D are adjacent angles]
80o+ ∠D = 180o
∠D = 180o– 80o= 100o