I am white when I am dirty, and black when I am clean. What am I?

A beggar on the street can make one cigarette out of every 6 cigarette butts he finds. After one whole day of searching and checking public ashtrays the beggar finds a total of 72 cigarette butts. How many cigarettes can he make and smoke from the butts he found?

You need to remove four matchsticks from the picture below and still need to maintain the following as in the original image.
1. 1st row contains 12 matchsticks
2. 2nd row contains 12 matchsticks
3. 1st column contains 12 matchsticks
4. 2nd column contains 12 matchsticks

Before Mount Everest was discovered, what was the highest mountain in the world?

I cry without having a voice.
I fly without having wings.
I bite without having teeth.
I whisper without a mouth.

Who am I?

In a science lab, a petri dish hosts a healthy colony of yeast for an experiment. Now every minute, all the yeast cells divide into two. At noon, there was just a single cell of yeast and at 1:22, the Petri dish was half full. Can you calculate when the dish will be full of yeast?

Solve below rebus puzzle?

1 + 9 + 8 = ?
considering, 28 + 8 + 92 = 10

In two decks of cards, what is the least amount of cards you must take to be *guaranteed* at least one four-of-a-kind?

What 4-letter word can be written forward, backward or upside down, and can still be read from left to right?