B.9 Always In(Ex)cluding Problems

Select 5 books from 12 so 2 books are

  1. always present
  2. never present
    1. \(^{12-2}C_{5-2} = ^{10}C_3 = 120\)
    1. \(^{12-2}C_5 = ^{10}C_5 = 252\)