The person who makes it has no need for it; the person who buys it has no use for it. The person who uses it can neither see nor feel it. What is it?

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

I cannot talk, but I always reply when spoken to. What am I?

A large water tank has two inlet pipes (a large one and a small one) and one outlet pipe. It takes 3 hours to fill the tank with the large inlet pipe. On the other hand, it takes 6 hours to fill the tank with the small inlet pipe. The outlet pipe allows the full tank to be emptied in 9 hours.

What fraction of the tank (initially empty) will be filled in 0.64 hours if all three pipes are in operation? Give your answer to two decimal places (e.g., 0.25, 0.5, or 0.75).

Place the numbers from 1 to 9 in the circles in a manner that each side of the triangle sums up to 17.

I have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?

A Shopkeeper sold a few chickens to four different customers on a particular day. It was such that each customer purchased half of the remaining chickens and half the chicken more.

Can you find out how many chicken were sold by the shopkeeper on that day if we tell you that the fourth customer bought a single chicken ?

I am a prime number.
The double of myself is equal to the square of me.

which number am I?

I never stop running even when I standstill. If I am formed by joining two identical bodies together, can you guess who I am?

I look at you, you look at me I raise my right, you raise your left. What am I?