(Q). In how many ways 15 students can be seated on 20 seats in a row ?
Sol:-
First select then arrange
15 seats can be selected out of 20 seats in ²⁰C₁₅ ways. Now 15 students can be arranged in these 15 seats in 15! ways.
Hence total number of ways = ²⁰C₁₅.15! Answer