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

You are in a room that has three switches and a closed door. The switches control three light bulbs on the other side of the door. Once you open the door, you may never touch the switches again. How can you definitively tell which switch is connected to each of the light bulbs?

What is the largest amount of money that you can possess in change and still if someone asks you for a dollar, you won't be able to give?

In particular commonwealth games, a female participant won silver and gold medals for a single event.

How can this be possible?

They are three errirs in this question. Can you find them?

I went to the bookshop and spent one-half of the money that was in my purse.

When I came out, I found that I had as many cents as I had dollars and half as many dollars as I had cents when I went in. Find the money in my purse when I entered the store.

Why do we preferably have round manhole covers and not square ones?

I have branches, but no fruit, trunk or leaves. What am I?

Thirty friends were on a hiking trip when they decided to enjoy the bonfire. They assembled for it and agreed to play a game. For that, they divided themselves into five teams with seven members each, forming five rows.

How did they manage to achieve this formation?

I can turn polar bears white.
I can make anyone dance.
I can make everyone cry
I can make your hands clap
I can make you thin.
I can make you smile.
I can make you smile

Can you guess the riddle?