What does this rebus riddle means ?

There are 2 sand hourglasses.

The small one can measure 5 hours and the large one can measure 7 hours.

How can we measure 16 hours with 2 sand hourglasses running together ?

Can you solve this picture maths equation?

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.

A seven-year-old kid challenged his classmates that he can make the number one disappear by adding something to it.

How can he do that?

Take away the whole and some still remain. What is it?

With the help clues provided in the image, can you find the 5 digit key?

My thunder comes before the lightning; my lightning comes before clouds; my rain dries all the land it touches. What am I?

Can you find out which options fits best with the missing column?

An American nightclub called 'The Coconut Grove' had a terrible fire in which over 400 people died. A simple design flaw in the building led to a high death toll. Subsequently, regulations were changed to ensure that all public buildings throughout the country eliminated this one detail, which proved deadly. What was it?