Below, you can see three complete equations and one incomplete. Based on the three complete ones, can you complete the incomplete one?

5 $ 4 $ 3 $ 9 = 4215
6 $ 9 $ 2 $ 6 = 3816
4 $ 7 $ 3 $ 3 = 1122
7 $ 2 $ 7 $ 4 = ____

I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

Here is a conversation between a couple who both are software professionals.

Husband: Hey! What is your date of birth?
Wife Replied Angrily: It is 591062400
Husband after doing some calculations replied, "Got It".

What is the DOB of the wife?

Find the next number in the series?
2, 12, 36, 80, 150, 252, 392, _ ?

You have two buckets of 11 liters and 6 litres.
How can you measure exactly 8 litres?

What number comes next in this sequence:
7 8 5 5 3 4 4 _

My first is in chocolate but not in ham. My second is in cake and also in jam. My third at tea time is easily found. Altogether, this is a friend who is often around. What is it?

Prove that the below maths equation is true.

8 + 8 = 91

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

Until it is measured, not known. Yet how you miss it when it has gone. What is it?