# How do you distinguish between whether should use a permutation and a combination for a given problem?

##### 1 Answer

Jul 5, 2015

Number of permutations of three letters from given five letters a, b, c, d, e is 60 as follows

abc,acb,bac,bca,cab,cba

abd,adb,bad,bda,dab,dba

abe,...

acd,..

ace,...

ade,...

bcd,..

bce,...

bde,...

cde,...

10 x 6 = 60

now number of combinations of five letters a,b,c,d,e taken three at a time is just ten, namely

abc,abd,abe,acd, ace,ade, bcd,bce, bde, cde.