Question

There are four postcards and five letter boxes. In how many ways can postcards be dropped if all of them are not dropped in the same letter box

• A. $620$
• B. $3120$
• C. $252$
• D. $1020$

Answer 1

The correct option is A $620$

When there is no restriction, each letter can be dropped in any $5$ of the letter boxes in $5$ ways.

Thus the total number of ways without restriction $=5\times 5\times 5\times 5=5^4$

However, in $5$ ways all the $5$ letters can be dropped in same letter box.

Thus required number of boxes$=5^4-5=620$