Which number should replace the question mark?

Eight of us go fourth, not back, to protect our king from an attack. What are we?

John was working on a Chemical Mixture whose weight comprise 90% liquid and 10% solid. The total weight of the mixture is 20 pounds. After a while, John noticed that some of the liquid evaporated and now the liquid comprises just 50% of the weight.

What is the weight of the chemical mixture now?

Peter wakes up daily to pick up his cycle and crosses the border between Spain and France daily with a bag on his shoulder. He is investigated daily by the officials but they don't find anything suspicious.

If we tell you that he is smuggling something what would it be?

It comprises roots which nobody sees,
Much taller than those trees,
Up and up it goes,
Still it never grows?

Our product and our sum always give the same answer. Who are we?

Two girls were born to the same mother, on the same day, at the same time, in the same month and year and yet they're not twins. How can this be ?

By removing one matchstick and adding one matchstick, can you convert the word "ZOO" to an animal name?

John buys rice at Rs.60/kg from Britain and then sells them at Rs.5/kg in India. As a result of this, he becomes a millionaire. How come?

What building has the most stories?