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

Can you divide the below-given shape into four equal and identical pieces?

I am not a bull but I have horns.
I am not an ass but I have a pack saddle.
Wherever I go, I leave silver behind me.

Who am I?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?

John speaks the truth only once a day in a week. Below are a few hints for you:

First: Days are Sunday, Monday and so on.
Second: One day he says, "I lie on Monday and Tuesday".
Third: On the next day, he says, "Today is either Thursday, Saturday or Sunday".
Fourth: On the next day, he says, "I lie on Wednesday and Friday".

Can you identify the day on which he speaks the truth?

What's the next in the series?

H, Be, F, S, Mn, Kr, In, Gd, Tl, ?

Find a 9-digit number, which you will gradually round off starting with units, then tenth, hundred etc., until you get to the last numeral, which you do not round off. The rounding alternates (up, down, up ...). After rounding off 8 times, the final number is 500000000. The original number is commensurable by 6 and 7, all the numbers from 1 to 9 are used, and after rounding four times the sum of the not-rounded numerals equals 24.

Two men are in a desert. They both have packs on. One of the guys is dead. The guy alive has his pack open, and the guy who is dead has his pack closed. What is in the pack?

Can you find out the missing number?

? 4 5 3 5

9 5 7 10 3

John went to meet his friend Jacob, but when he was about to reach the main gate, John notices that Jacob had a mighty dog who was fastened to the tree. The chain is long enough that it allows the dog to reach the main gate.

How can the John reach the main gate?