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

In a school, there are four subjects. Seventy percent of students study English, seventy five percent of students study Science, eighty five percent of students study Mathematics and eighty percent of students study Spanish.

Can you calculate the percentage of students that study all four subjects?

It is always ahead of me yet I can never see it. What is it?

Spelt forwards I’m what you do every day, spelled backwards I’m something you hate. What am I?

what does the below rebus mean?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?

They came out at night, without being called.
They lost at day, without being stolen.
Who are they?

In the picture that is attached with this question, you can find a square which comprises of four little squares inside it. Consider this square to be made with matchsticks. You have to remove two matchsticks such that only two squares remain instead of five.How will you do it ?

This is a classy optical illusion puzzle. What more you can find except bricks in the image below?

What number does the below three word/phrase identify and why?
1. Deck
2. Year
3. Singing in the Rain