Question

A has 5 books and B has 4 books. In how many ways can they exchange their books keeping the same no. of books they had first

• A. 126
• B. 125
• C. 120
• D. 96

Answer 1

Answer: (B)

125

Answer 2

Method I: No. of ways =

$5C_{1}.^{4}C_{1} +^{5}C_{2}.^{4}C_{2} +^{5}C_{3}.^{4}C_{3} +^{5}C_{4}.^{4}C_{4}$.

Method II: No. of ways = ⁹C₅ – 1

= 125