text: It's not quite right.  You can tweak the formula to make it work.\n\nYours: \nnC(n-b) + (n-1)C(n-b-1) + (n-2)C(n-b-2) + ... + bC0 = (n+1)C(n+b)\nwhere  0 (</=) b (</=) n\n\nActual: nC(n-b) + (n-1)C(n-b-1) + (n-2)C(n-b-2)+...+(n-b)C0= (n+1)Cb\n\nTry that and see if it looks OK