What is full of holes but still holds water?

Pronounced as one letter,
And written with three,
Two letters there are,
And two only in me.
I am double, I am single,
I am black blue and grey,
I am read from both ends,
And the same either way.

Name a key which is hardest to turn.

Take away the whole and some still remain. What is it?

A generous owner of a company decided to give a bonus of $45 to every man and $60 to every woman on his birthday. But only one-ninth of the men and one-twelfth of the women were present to take the bonus.

Can you calculate the amount of money the owner spent if there were 3552 employees?

If all Goats are Sheep and all Sheep are Deers, are all Deers Goats ?

How can you make the following equation true by drawing only one straight line: 5+5+5=550 ?

Find the value of "?" in the ball picture Riddle below:

A train leaves from point A for point B at 80 mph. After half an hour, another train leaves from point B for point A at 60 mph.

Which of the trains will be farther from point A when they meet ?

It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?