Chapter 13 - Counting and Probability - 13.2 Permutations and Combinations - 13.2 Asses Your Understanding - Page 855: 24

The ordered arrangements: $ab,ac,ad,ae$ $ba,bc,bd,be$ $ca,cb,cd,ce$ $da,db,dc,de$ $ea,eb,ec,ed$ $P(5,2)=20$.

The ordered arrangements: $ab,ac,ad,ae$ $ba,bc,bd,be$ $ca,cb,cd,ce$ $da,db,dc,de$ $ea,eb,ec,ed$ We know that $P(n,r)=n(n-1)(n-2)...(n-k+1).$ Also $P(n,0)=1$ by convention. Hence $P(5,2)=5\cdot4=20$.