Given,
Cr−1n:Crn:Cr+1n=1:5:20
Now using the formula Cr−1nCrn=rn−r+1 we get,
.Cr−1nCrn=rn−r+1=15
⇒n−6r+1=0.........(1)
And CrnCr+1n=r+1n−r=520
⇒n−5r−4=0..........(2)
Solving above equations we get,
n=29&r=5
So, the coefficient of forth term will be C3n=C329=3×2×129×28×27=3654