Which number should replace the question mark?

Decode the below four famous ciphers:

8P of the SS
1M of the E
1S of the SS
60M in 1H

A tree doubled in height each year until it reached its maximum height over the course of ten years. How many years did it take for the tree to reach half its maximum height?

Make the seating arrangement of 10 children in such a way that there are 5 rows with 4 children in each row.

Two guards were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?"

How did he know his companion was smiling?

What time is it when an elephant sits on a fence?

What English word has three consecutive double letters?

I am a word that begins with the letter “i.” If you add the letter “a” to me, I become a new word with a different meaning, but that sounds exactly the same. What word am I?

At a party, there are five people and a whole round cake lying at the centre of the table. Only four people will make a cut and take their piece and the last one will get the remaining piece on the table. How can they make sure that everyone gets a 1/5th of the piece?

What can jump higher than a building?