Can you replace the question mark with the correct number in the below table sequence?

What is the next number in this series?

4,12,84,3612....

Count the number of times the letter "F" appears in the following paragraph:

FAY FRIED FIFTY POUNDS OF
SALTED FISH AND THREE POUNDS
OF DRY FENNEL FOR DINNER FOR
FORTY MEMBERS OF HER FATHER'S FAMILY.

Bopanna was working on a murder case when he received a message about the location of the murder weapon.
The message was encrypted as below.

"deb mitciv eht edisni neddih si nopaew redrum ehT"

Tell me the location of the murder weapon.

Two prisoners Jack and Jill are locked in a cell.
There is open window approx 40 feet above the ground of the cell.
They are never able to reach there.

Then they plan to escape by a tunnel and they start digging out.
After digging for more than 20 days, Jill comes with the different plan and they escaped.

what was the plan ?

Can you think of a smallest +ve number such that if we shuffle the digits of the number, the new number becomes double the original number?

There are a number of books on a shelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?

John was driving a bus and got stuck when passing through an underpass. Then came Jacob with a needle and got the bus out of the underpass.

What did the boy do?

Complete the series by replacing "?" with the correct number.

ST ND RD TH '?'

What is both possible and impossible at the same time?