Answer : Given: ⁿCr–1 = ⁿC3r

To find: r=? We know that:

ⁿCr = ⁿCn-r

⇒ nCr–1 = nCn-(r-1)

⇒ nCr–1 = nCn-r+1

⇒ nCn-r+1= nC3r

⇒n-r+1=3r

⇒4r=n+1