If ²² P_ r+1 : ²⁰ P_ r+2 =11: 52 , then r is equal to

Step-by-step solution

²² P_ r+1 : ²⁰ P_ r+2 =11: 52 or (21-r)(20-r)(19-r)=52 2 21 (21-r)(20-r)(19-r)=14 13 12 aligned & (21-r)(20-r)(19-r)=(21-7)(20-7)(19 & -7) aligned r=7