Never seen Yd before!! ...Y stands for income, as far as I know (as in YED = income elasticity of demand). AD is always AD. But AD = Yd (d for demand) because the demand is as high as the consumers can afford it to be.
I take it I= Investment, G=Government spending, E=equilibrium income.
C = 20 + 0.8 x (Y-T) = 20 + 0.8Y-0.8(0.25Y)
At equilibrium, E=Y, so Y= C + I + G
Y = C + I + G = 20 + 0.8Y - 0.2Y + 50 + 90
Y - 0.6Y = 160
0.4Y = 160
Y = 400.
That's the best I can do...