If all four letters are different then the number of words 5C4×4!=120 If two letters are R and other two different letters are chosen from B,A,C,K then the number of words =4C2×
4!
2!
=72 If two letters are A and other two different letters are chosen from B,R,C,K then the number of words =4C2×
4!
2!
=72 If word is formed using two R 's and two A 's then the number of words =
4!
2!2!
=6 Therefore, the number of four-letter words that can be formed =120+72+72+6=270