Why is it very easy to weigh a fish?

Two boys wish to cross a river. The only way to get to the other side is by boat, but that boat can only take one boy at a time. The boat cannot return on its own, there are no ropes or similar tricks, yet both boys manage to cross using the boat.

How?

When you say goodbye, I am what you need to tie.
I come as a couple and go wherever you run.
Who am I?

If "P" means "-", "Q" means "/", "R" means "+", and "S" means "*" then find the value of the following:

21 Q 7 R 9 S 10 P 13

Queen Elizabeth organised a royal party.

To Avoid uninvited guest royal family set a password.
Jack (an uninvited person) plan to enter the party. He stand nearby the door.

First guest comes, the security person said 'twelve' and guest replied with six.
Second guest comes , the security person said 'six' and guest replied with 'three'.

Jack thought is enough and he walked to the entry point. The security person said 'eight' , Jack replied smilingly 'four'.
He was immediately thrown out of the party. why ?

Can you find out the missing number in the grid?

What has a bottom at the top?

What month of the year has 28 days?

Can you find the next number in the below sequence
3 , 7 , 10 , 11 , 12 , 17 ?

using four eights (8) and a one (1) and one mathematical symbol , create the number 100