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Equation 1, D , I = 30
I = k/D^2
30 = k/D^2
k = 30 D^2 ——- (1)
Equation 2, D-2, I = 120
I = k/(D-2)^2
120 = k/(D-2)^2 —— (2)
Sub (1) to (2), you will get
3D^2 -16D +16 = 0
Find D, D = 4
sub D into (1) and (2),
both equation you will get k = 480
Answering the question, sub D=4 and k= 480
I = k/(D-3.5)^2
I =480/0.5^2
I = 1920