The number of four letter words that can be formed using the letters of the word MIXTURE which contains the letter X is 
 

Consider 4 places. The letter X can be placed in one place in 4 ways.

Remaining 3 places can be filled using M,I,T,U,R,E in ${ }^6{P}_3$ ways

Number of 4 letter words containing the letter X=${ }^6{P}_3\times 4=480$