No matter how little or how much you use me, you change me every month. What am I?

Can you solve this amazing hard number series problem?
7, 8, 10, 12, 16, 18 ?

I have 50 feet long of cloth, I need to create 50 handkerchiefs each size of 1 foot. I take one minute to cut a 1-foot handkerchief. So how long will it take to cut 50 handkerchiefs?

Find three whole, positive numbers that have the same answer when multiplied together as when added together.

In the given figure, move three matchsticks so that the resulting figure contains two rectangles.

A Child was born in Lahore, Pakistan. Still, the child is not a Pakistani citizen. why?

John is found dead in his office at his desk. The police have narrowed the suspects down to three people: Mrs. Jacob, John’s wife Hena and his business partner Jason . All three visited John on the day of his murder, but all three provide the police with stories of explanation as to the reason for their visit.
Police found John with his wrist watch still on his right arm, a torn up picture of his wife laying on the floor beside the trash can, and an ink pen in his right hand. On the desk, the police found a name plate, a telephone that was off the hook, and a personal calendar turned to the July 5th page with 7B91011 written on it. After examining this evidence, the police knew their suspect. Who was it ?

Before reading ahead, you must know the fact that only one of the people here is telling the truth.

A says that B is lying.
B says that C is lying.
C says both A and B are lying.

Can you find out who is speaking the truth?

A father's child, a mother's child, yet no one's son.

Who am I?

A pond had 60 fish, out of which, all but 20 died.
How many fishes are now present in that pond?