I am white when I am dirty, and black when I am clean. What am I?

Spot 5 differences in two Christmas tree below:

0 0 0 = 6
1 1 1 = 6
2 2 2 = 6
3 3 3 = 6
4 4 4 = 6
5 5 5 = 6
6 6 6 = 6
7 7 7 = 6
8 8 8 = 6
9 9 9 = 6

You can use any mathematical symbols in the space provided to make all above algebraic expressions true.

It has a long neck, A name of a bird, Feeds on cargo of ships, It's not alive,

What is it?

There is so small cabin inside a deep forest.
All the forest burned out to worst possible but the inside of the cabin is still the same.
However all the person inside the cabin die.

How come ?

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

Here is a mathematical expression that also secretly tells a movie name. What movie is that?

On a bus, there is a 26-year-old pregnant lady.
A 30-year-old policeman.
A 52-year-old random woman.
And the 65-year-old driver.

Who is the youngest?

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?

What number comes next in the series?

5 15 5 18 5 24 14 20 _