21 buses are running between two places P and Q. In how many ways can a person go from P to Q and return by a different bus?	Choose the correct choice from the following answers
A) None of these	B) 420	C) 212	D) 224	E) 240
Explanation :   He can go in any bus out of the 21 buses.   Hence He can go in 21 ways.   Since he can not come back in the same bus that he used for travelling,   He can return in 20 ways.   Total number of ways = 21 x 20 = 420. Answer : Option 420
<result>B</result>
21 buses are running between two places P and Q. In how many ways can a person go from P to Q and return by a different bus?