Question 974074


Hello,


I remember this from Discrete Math. 


15 is a superscript of C and 3r, r+3 are subscripts of C. If 15C3r = 15Cr+3, then we follow the formula:


nCx = nCy where x = y OR x + y = n.


Let n represent 15, x = 3r and y = r +3:


3r = r + 3 and 3r + r + 3 = 15


3r = r + 3 ---> 3r - r =3 then 2r = 3 which is r = 3/2


3r + r + 3 = 15  ---> 4r = 12 which is r = 3


r cannot be a fraction since this will give us 1.5 therefore the answer will be r = 3.