In how many ways can you sit 6 people on a bench if 3 of them must sit together?
Options: A) 144 B) 160 C) 400 D) 320 E) 420
In such questions, always tie the person that have to sit together. So we have effectively 3+1= 
<gadget id="calculator">3+1</gadget>
<output>4</output>
4 persons to arrange.   They can be arranged in 4! ways.   Now the 3 persons can themselves be arranged in 3! ways.   Total ways: 4!*3!= 144.   144 is the answer.
<result>A</result>
In how many ways can you sit 6 people on a bench if 3 of them must sit together?