The centre of the third figure is empty. What digit should fit inside?

There was a man he lives in a hotel each morning he presses the first floor button each evening if there is a person in the elevator he asks him to press the 10 floor button if there is no one in the elevator he takes the stairs why.

When you stop to look, you can always see me. But if you try to touch me, you can never feel me. Although you walk towards me, I remain the same distance from you. What am I?

Take one out and scratch my head, I am now black but once was red. What am I?

There are two insects on a tile. Insect X is sitting on one side of the tile (point A) and Insect Y is sitting opposite on the other side of the tile (point B). Now both of them decide to change their position and thus X starts crawling to point B and Y starts crawling to point A. When they meet and pass each other in between, X takes 20 seconds to reach B and Y takes just 5 seconds to reach A.

Can you calculate the total time each of the insects took to change their positions?

What is the centre of gravity?

When we see 2 but call 10?

Using eight eights and addition only, can you make 1000?

My cousin lives in one story house in Antarctica. His house is made the of-of sandstone. what is the color of stairs?

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?