Easy Puzzles 

Workings

Hint
Fill the 9-litre container first.

Answer
Here are the contents of the containers after each stage:

A = 9 litres, B = 4 litres
A     B
9     0     fill A
5     4     fill B from A (leaving 5 litres in A)
5     0     empty B
1     4     fill B from A (leaving 1 litre in A)
1     0     empty B
0     1     fill B from A
9     1     fill A
6     4     fill B from A (only 3 more litres fit in B)
6     0     empty B

This leaves 6 litres in container A.

Workings

Hint
Some might argue that this isn't a puzzle.

Answer
Half way, after which it would be running out!

Workings

Hint
Where on Earth can you end up back where you start?

Answer
White.

They were at the north pole and saw a polar bear.

Workings

Hint
A very, very easy puzzle?

Answer
5.

Naturally!