The crew of an 8 oar boat is to be choosen from 12 men of whom 3 can row always on the stroke side only. The number of ways in which the crew can be arranged is 

Number of ways of arranging 4 people on non stroke side $=9_{{\mathrm{C}}_4}\times 4!$

Number of ways of arranging 4 people from remaining 8 people on stroke side $=8_{{\mathrm{C}}_4}\times 4!$ 

Total no of arrangements $9_{{\mathrm{C}}_4}\times 8_{{\mathrm{C}}_4}\times 4!\times 4!$