What goes up but never comes down?

Solve the mathematical equation in the picture below.

What is 3/7 chicken, 2/3 cat and 2/4 goat?

What brings you down but never up?

Twenty frogs are sitting on a log floating on the surface of a river. Two of them decide to jump off into the water.

How many frogs are there on the log at this moment?

Can you convert the number zero(0) into a positive number using just one mathematical operator ?

Can you identify the Oscar-winning movie from the rebus below?

I possess a head

I flaunt a tail

But I don't have mouth or legs

Can you find out who I am?

This was asked in an interview with Coa-Cola.

The interviewer has given me 100 marbles(50 white and 50 black) and two empty boxes.
He then told me that he will leave the room and i need to place all the marbles in two boxes.

And When he come back, he will draw a marble from any of the two box and if the marble is white I will be hired.

Also
* No box can be empty.
* All 100 marbles must be placed in one of the two boxes.

So what should I have done?

Buy me, smell me or deliver me, I do not change.

Who am I?