How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?

In a mathematical quiz, you are asked the following question:

Use three 9s in a mathematical expression without dividing or multiplying to form the number one.

Can you answer the question?

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

Why is the Hole below a Lock?

What do you see in the given picture?

By using 'because' continuously three times, can you make an English statement ?

Before the start of the car race, John and Jacob have the same amount of fuel in their car. With this fuel, John can drive for 4 hours while Jacob can drive five hours.
After a time they realize that the amount of fuel left in John's car is 1/4th of the fuel in Jacob's.

For how long they are racing?

Krish is 45 years old and his mother Crishtina is 73 years old.

Can you find out how many years ago, his mother was three times his age?

Two guards were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?"

How did he know his companion was smiling?

What does this mathematical rebus means ?