Consonants →{b,c,d,f,g,h}
Vowels →{a,e,i,o,u}
× × × ×
Case I: If word begins with consonants, then
Number of words =(6C4×4!)×(5C3×3!)
=360×60=21600
Case II: If word begins with vowels, then
Number of words =(5C4×4!)×(6C3×3!)
=120×120=14400
Total count =21600+14400=36000
⇒10k=36000
⇒k=3600