Independent solution

How to solve this Combinatorics question

Setup

Setup

Choose which three of the five columns are occupied, then assign a distinct row to each chosen column.

(53) ways to choose occupied columns\binom53\ \text{ways to choose occupied columns}

Model

Model

After the column set is fixed, there are 6×5×4 injective row assignments.

654 ways to assign distinct rows6\cdot5\cdot4\ \text{ways to assign distinct rows}

Compute

Compute

Multiplying 10 column choices by 120 row assignments gives 1200 valid placements.

(53)(6)(5)(4)=10(120)=1200\binom53(6)(5)(4)=10(120)=1200

Answer

Answer

The number of arrangements is 1200, corresponding to choice C.

1200(C)\boxed{1200\quad\text{(C)}}