If eleven plus two equals one, what does nine plus five equal?

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

Can you name three consecutive days without using the words Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday?

Can you solve this picture maths equation?

Do You See It?

I am a white box having no key or lid.
Yet there is a golden treasure inside it.

What Am I?

Can you spot what's wrong with the following sentence?

PETER WAS ANGRY AT
HIMSELF BECAUSE HE
ACCIDENTALLY LEFT THE
THE KEYS TO HIS CAR
AT HIS FRIEND'S HOUSE.

Can you name any English verb that becomes its past tense by simply rearranging the alphabets ?

The dirtier I get, the whiter I look. Who am I?

He digs out tiny caves and stores gold and silver in them. He also built bridges of silver and made crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs his help, yet many people are afraid to let him help them. Who is he?