6 friends are living in 6 different flats of an apartment. Each of them was allotted a parking spot to park their cars in the ground floor. But they used to park their cars randomly in any of the 6 parking spots. In how many ways can they park their cars so that exactly 4 people park their cars in the spots allotted to them?  Pick one
A) 15  B) 21  C) 35  D) 42  E) 49
6C4 = 15   The remaining 2 people in each case will be parked in each other's parking spots.   The answer is 15.
<result>A</result>
6 friends are living in 6 different flats of an apartment. Each of them was allotted a parking spot to park their cars in the ground floor. But they used to park their cars randomly in any of the 6 parking spots. In how many ways can they park their cars so that exactly 4 people park their cars in the spots allotted to them?