What is the centre of gravity?

what does the below rebus means?

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

John drives to his office at 20km/hr. After reaching the office, he realizes that today is a holiday. He went back at an average speed of 30km/hr. Discounting the time spent in the stoppage what was the average speed of his journey?

There is a square of a particular number which when doubled, becomes 7 more than its quarter.

Can you find the number?

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?

I am a type of word that is spelt incorrectly or used differently, on purpose for humour. What am I?
Hint 1: You can find me in a joke.
Hint 2: I am a play on words.

John and his wife were living in a rural place. On a particular day, John's wife fell ill and he called the local doctor. When the doctor picked up, he said, "Doctor, my wife is ill. She might have appendicitis."

"This can't be possible! I took out her appendix two years ago myself," the doctor explained.

When diagnosed, John's wife was found to have appendicitis. How can this be possible?

Why do Chinese men eat more rice than Japanese men do ?

What will be the best approach to finding all the prime numbers less than 75 that leave an odd reminder when we divide them with 5?